Asymmetric semiconductor memory device having electrically floating body transistor

ABSTRACT

Asymmetric, semiconductor memory cells, arrays, devices and methods are described. Among these, an asymmetric, bi-stable semiconductor memory cell is described that includes: a floating body region configured to be charged to a level indicative of a state of the memory cell; a first region in electrical contact with the floating body region; a second region in electrical contact with the floating body region and spaced apart from the first region; and a gate positioned between the first and second regions, such that the first region is on a first side of the memory cell relative to the gate and the second region is on a second side of the memory cell relative to the gate; wherein performance characteristics of the first side are different from performance characteristics of the second side.

CROSS-REFERENCE

This application is a continuation application of co-pending applicationSer. No. 15/356,540, filed on Nov. 19, 2016, which is a continuationapplication of application Ser. No. 14/591,454, filed on Jan. 7, 2015,now U.S. Pat. No. 9,524,970, which is a continuation application ofapplication Ser. No. 13/244,899, filed on Sep. 26, 2011, now U.S. Pat.No. 8,957,458, all of which applications and patent are herebyincorporated herein by reference in their entireties and to whichapplications we claim priority under 35 USC § 120. Application Ser. No.13/244,899 claims the benefit of U.S. Provisional Application No.61/466,940, filed on Mar. 24, 2011 and titled “An Asymmetric MemoryDevice Comprising of Electrically Floating Body Transistor”, whichprovisional application is hereby incorporated herein, in its entirety,by reference thereto, and to which provisional application we claimpriority under 35 USC § 119.

Application Ser. No. 13/244,899 claims the benefit of U.S. ProvisionalApplication No. 61/471,712, filed on Apr. 5, 2011 and titled “AnAsymmetric Memory Device Comprising of Electrically Floating BodyTransistor”, which provisional application is hereby incorporatedherein, in its entirety, by reference thereto and to which provisionalapplication we claim priority under 35 USC § 119.

Application Ser. No. 13/244,899 claims the benefit of U.S. ProvisionalApplication No. 61/485,081, filed on May 11, 2011 and titled “AsymmetricSemiconductor Device Having Electrically Floating Body Transistor”,which provisional application is hereby incorporated herein, in itsentirety, by reference thereto and to which provisional application weclaim priority under 35 USC § 119.

FIELD OF THE INVENTION

The present invention relates to semiconductor memory technology. Morespecifically, the present invention relates to an asymmetricsemiconductor memory device having an electrically floating bodytransistor.

BACKGROUND OF THE INVENTION

Semiconductor memory devices are used extensively to store data.Volatile memory such as Static and Dynamic Random Access Memory (SRAMand DRAM, respectively) are widely used in many applications. However,volatile memory loses its data when power is not continuously supplied.

DRAM based on the electrically floating body effect has been proposed(see, for example “A Capacitor-less 1T-DRAM Cell”, S. Okhonin et al.,pp. 85-87, IEEE Electron Device Letters, vol. 23, no. 2, February 2002(“Okhonin-1”), which is incorporated by reference herein in its entiretyand “Memory Design Using One-Transistor Gain Cell on SOI”, T. Ohsawa etal., pp. 152-153, Tech. Digest, 2002 IEEE International Solid-StateCircuits Conference, February 2002) (“Ohsawa-1”), which is incorporatedby reference herein in its entirety). Such a memory eliminates thecapacitor used in conventional one transistor, one capacitor (1T/1C)memory cell, and thus is easier to scale to smaller feature size. Inaddition, such memory allows for a smaller cell size compared to theconventional 1T/1C memory cell. Both Okhonin-1 and Ohsawa-1 describe aDRAM memory cell comprising a single standard metal-oxide-semiconductorfield effect transistor (MOSFET) having a gate terminal, twosource/drain terminals, and a floating body fabricated usingsilicon-on-insulator (SOI) complimentary metal-oxide-semiconductor(CMOS) technology. Ohsawa-1 further describes a current mirror senseamplifier which compares the current of a sensed cell to the average oftwo reference cells, one written to logic-0 and the other written tologic-1.

It would be desirable to provide memory devices having improved readoperations to what is currently known.

It would further be desirable to provide such memory devices having asize that is not prohibitively larger than comparable volatile memorydevices.

The present invention meets all of the above desires and more.

SUMMARY OF THE INVENTION

In one aspect of the present invention, an asymmetric, bi-stablesemiconductor memory cell is provided that includes: a floating bodyregion configured to be charged to a level indicative of a state of thememory cell; a first region in electrical contact with the floating bodyregion; a second region in electrical contact with said floating bodyregion and spaced apart from the first region; and a gate positionedbetween the first and second regions, such that the first region is on afirst side of the memory cell relative to the gate and the second regionis on a second side of the memory cell relative to the gate; whereinperformance characteristics of the first side are different fromperformance characteristics of the second side.

In at least one embodiment, the memory cell includes a gap region on asurface of the floating body region, the gap region located between thefirst region and the gate.

In at least one embodiment, the memory cell includes a substrate and aburied layer in the substrate, wherein the substrate is separated fromthe floating body region by the buried layer.

In at least one embodiment, the memory cell includes a word lineterminal electrically connected to the gate; a bit line terminalelectrically connected to the first region; a source line terminalelectrically connected to the second region; a buried well terminalelectrically connected to the buried layer; and a substrate terminalelectrically connected to the substrate.

In at least one embodiment, the first region has a first conductivitytype selected from a p-type conductivity type and an n-type conductivitytype; the floating body region has a second conductivity type selectedfrom the p-type and n-type conductivity types, the second conductivitytype being different from the first conductivity type; and the secondregion has the first conductivity type.

In at least one embodiment, the memory cell includes a substrate havingthe first conductivity type; and a buried layer in the substrate, theburied layer having the second conductivity type, wherein the substrateis separated from the floating body region by the buried layer.

In at least one embodiment, the memory cell includes an insulating layerinsulating the gate from the floating body region.

In at least one embodiment, the floating body region has a firstconductivity type selected from a p-type conductivity type and an n-typeconductivity type; the first region has a second conductivity typeselected from the p-type and n-type conductivity types, the secondconductivity type being different from the first conductivity type; andthe second region had the first conductivity type.

In at least one embodiment, the memory cell includes the second regionhas a first conductivity type selected from a p-type conductivity typeand an n-type conductivity type; and the first region has a secondconductivity type selected from the p-type and n-type conductivitytypes, the second conductivity type being different from the firstconductivity type.

In at least one embodiment, the memory cell includes asilicon-on-insulator substrate; and a buried insulator layer, whereinthe buried insulator layer insulates the silicon-on-insulator substratefrom the floating body region.

In at least one embodiment, the memory cell is configured for use as areference cell, wherein the asymmetric semiconductor memory cell furthercomprises a third region in electrical contact with the floating bodyregion, the third region having the second conductivity type.

In at least one embodiment, the third region is located between the gateand the second region.

In at least one embodiment, the second region is electrically connectedto a gate of a switching transistor to configure connectivity of gatesin a field programmable logic array (FPGA).

In at least one embodiment, the memory cell is configured to function asa configuration memory, wherein the second region is electricallyconnected to a gate of a switching transistor that is connected tointerconnect lines connected to a field programmable logic array (FPGA);and an inverter and a p-channel metal-oxide-semiconductor (PMOS)transistor are connected to one of the interconnect lines to restorevalues of signals passed between the interconnect lines.

In at least one embodiment, the memory cell is configured for use as areference cell, wherein the asymmetric semiconductor memory cell furthercomprises a third region in electrical contact with the floating bodyregion, the third region having the second conductivity type.

In at least one embodiment, the third region is located between the gateand the second region.

In at least one embodiment, the memory cell is useable as a referencecell by applying an intermediate potential between a first potentialindicative of a logic-0 state and second potential indicative of alogic-1 state to the floating body region through the second region.

In at least one embodiment, the memory cell includes a substrate; and aburied layer in the substrate, wherein the substrate is separated fromthe floating body region by the buried layer; wherein the second regionis electrically connected to an operational amplifier and theoperational amplifier is further electrically connected to the buriedlayer, forming a feedback loop to the cell.

In at least one embodiment, the memory cell includes a substrate; and aburied layer in the substrate, wherein the substrate is separated fromthe floating body region by the buried layer; wherein the second regionis electrically connected to an input terminal of a CMOS inverter, andan output terminal of the CMOS inverter is electrically connected to theburied layer.

In at least one embodiment, the memory cell includes a substrate; and aburied layer in the substrate, wherein the substrate is separated fromthe floating body region by the buried layer; wherein the cell isconnected in a mixed-signal feedback loop.

In at least one embodiment, the mixed-signal feedback loop comprises thesecond region being electrically connected to an analog-to-digitalconverter a digital controller and a digital to analog converter, thedigital to analog converter being electrically connect to the buriedlayer.

In at least one embodiment, the mixed-signal feedback loop comprises a1-bit comparator block used to quantize a potential of the floating bodyregion.

In at least one embodiment, the memory cell includes a substrate;wherein the cell comprises a three-dimensional memory structure having afin that extends substantially perpendicular to, and above a top surfaceof the substrate.

In at least one embodiment, the floating body region, the first regionthe second region and the gate are formed in the fin.

In at least one embodiment, the gate comprises two gates, the gatesbeing formed on opposite sides of the floating body region.

In at least one embodiment, the gate wraps around three sides of thefloating body region.

In another aspect of the present invention, a semiconductor memory arrayis provided, including: a plurality of asymmetric semiconductor memorycells as described above, arranged in a matrix of rows and columns.

In another aspect of the present invention, an asymmetric semiconductormemory cell is provided that includes: a floating body region configuredto be charged to a level indicative of a state of the memory cell; afirst region in electrical contact with the floating body region; anelectrode electrically connected to the floating body region, whereinthe electrode forms a Schottky contact with the floating body region;and a gate positioned between the first region and the electrode.

In at least one embodiment, the memory cell includes a substrate; and aburied layer in the substrate, wherein the substrate is separated fromthe floating body region by the buried layer.

In at least one embodiment, the memory cell includes a word lineterminal electrically connected to the gate; a bit line terminalelectrically connected to the electrode; a source line terminalelectrically connected to the first region; a buried well terminalelectrically connected to the buried layer; and a substrate terminalelectrically connected to the substrate.

In at least one embodiment, the first region has a first conductivitytype selected from a p-type conductivity type and an n-type conductivitytype; and the floating body region has a second conductivity typeselected from the p-type and n-type conductivity types, the secondconductivity type being different from the first conductivity type.

In at least one embodiment, the memory cell includes a substrate havingthe second conductivity type; and a buried layer in the substrate, theburied layer having the first conductivity type, wherein the substrateis separated from the floating body region by the buried layer.

In at least one embodiment, the memory cell includes an insulating layerinsulating the gate from the floating body region.

In at least one embodiment, the memory cell includes a gap region on asurface of the floating body region, the gap region located between theelectrode and the gate.

In at least one embodiment, the memory cell includes a substrate;wherein the cell comprises a three-dimensional memory structure having afin that extends substantially perpendicular to, and above a top surfaceof the substrate.

In at least one embodiment, the floating body region, the first region,the electrode and the gate are formed in the fin.

In at least one embodiment, the gate comprises two gates, the gatesbeing formed on opposite sides of the floating body region.

In at least one embodiment, the gate wraps around three sides of thefloating body region.

In another aspect of the present invention, a semiconductor memoryarray, is provided, including: a plurality of asymmetric semiconductormemory cells as described above, arranged in a matrix of rows andcolumns.

In another aspect of the present invention, a method of operating amemory array having rows and columns of memory cells assembled into anarray of the memory cells, wherein at least one of the memory cells isan asymmetric memory cell having first and second sides, whereinperformance characteristics of the first side are different fromperformance characteristics of the second side, each memory cell havinga floating body region; is provided, wherein the method includes:accessing at least one of the asymmetric cells; and performing anoperation on the at least one asymmetric cell.

In at least one embodiment, the array comprises a plurality of theasymmetric cells, each asymmetric cell comprising a gate; a word lineterminal electrically connected to the gate; a bit line terminal; asource line terminal; a floating body region; a buried layer; a buriedwell terminal electrically connected to the buried layer; a substrate;and a substrate terminal electrically connected to the substrate, themethod further comprising performing a holding operation on the memorycells of the array.

In at least one embodiment, the performance of a holding operationcomprises: applying a positive back bias to the buried well terminal;applying zero bias to the word line terminal; applying zero bias to thebit line terminal; applying zero bias to the source line terminal; andapplying zero bias to the substrate terminal.

In at least one embodiment, the method includes monitoring cell currentin at least one of the cells; and modulating an amount of potentialapplied to the buried well terminal connected to the at least one of thecells by an amount functionally related to the cell current monitored inthe at least one of the cells.

In at least one embodiment, the array comprises a plurality of theasymmetric cells, each asymmetric cell comprising a gate; a word lineterminal electrically connected to the gate; a bit line terminal; asource line terminal; a floating body region; a buried layer; a buriedwell terminal electrically connected to the buried layer; a substrate;and a substrate terminal electrically connected to the substrate, themethod further comprising performing a read operation on a selectedmemory cell of the array.

In at least one embodiment, the performance of a read operationcomprises: applying zero bias to the word line terminal electricallyconnected to the selected cell; applying a positive bias to the bit lineterminal electrically connected to the selected cell; applying zero biasto the source line terminal electrically connected to the selected cell;applying a zero or positive bias to the buried well terminalelectrically connected to the selected cell; and applying zero bias tothe substrate terminal electrically connected to the selected cell.

In at least one embodiment, the method includes applying zero volts toall word line terminals not electrically connect to the selected cell;applying zero volts to all bit line terminals not electrically connectto the selected cell; and applying zero volts to all source terminalsnot electrically connect to the selected cell.

In at least one embodiment, the method includes providing a sensingcircuit connected to the array; and determining a state of the selectedmemory cell by sensing through the bit line terminal electricallyconnected to the selected cell, using the sensing circuit.

In at least one embodiment, the array comprises a plurality of theasymmetric cells, each asymmetric cell comprising a gate; a word lineterminal electrically connected to the gate; a bit line terminal; asource line terminal; a floating body region; a buried layer; a buriedwell terminal electrically connected to the buried layer; a substrate;and a substrate terminal electrically connected to the substrate, themethod further comprising performing a write logic-1 operation on aselected memory cell of the array.

In at least one embodiment, the performance of a write logic-1 operationcomprises: applying a positive voltage to the word line terminalelectrically connected to the selected cell; applying a positive voltageto the bit line terminal electrically connected to the selected cell;applying zero voltage to the source line terminal electrically connectedto the selected cell; applying a zero or positive bias to the buriedwell terminal electrically connected to the selected cell; and applyingzero bias to the substrate terminal electrically connected to theselected cell.

In at least one embodiment, the array comprises a plurality of theasymmetric cells, each asymmetric cell comprising a gate; a word lineterminal electrically connected to the gate; a bit line terminal; asource line terminal; a buried layer; a buried well terminalelectrically connected to the buried layer; a substrate; and a substrateterminal electrically connected to the substrate, the method furthercomprising performing a write logic-1 operation on a selected memorycell of the array, via a band-to-band tunneling mechanism.

In at least one embodiment, the performance of a write logic-1 operationcomprises: applying a negative voltage to the word line terminalelectrically connected to the selected cell; applying a positive voltageto the bit line terminal electrically connected to the selected cell;applying zero voltage to the source line terminal electrically connectedto the selected cell; applying a zero or positive bias to the buriedwell terminal electrically connected to the selected cell; and applyingzero bias to the substrate terminal electrically connected to theselected cell.

In at least one embodiment, the array comprises a plurality of theasymmetric cells, each asymmetric cell comprising a gate; a word lineterminal electrically connected to the gate; a bit line terminal; asource line terminal; a buried layer; a buried well terminalelectrically connected to the buried layer; a substrate; and a substrateterminal electrically connected to the substrate, the method furthercomprising performing a write logic-0 operation on the array.

In at least one embodiment, the performance of a write logic-0 operationcomprises: applying zero voltage to the word line terminal electricallyconnected to the selected cell; applying zero voltage to the bit lineterminal electrically connected to the selected cell; applying anegative voltage to the source line terminal electrically connected tothe selected cell; applying a zero or positive bias to the buried wellterminal electrically connected to the selected cell; and applying zerobias to the substrate terminal electrically connected to the selectedcell.

In at least one embodiment, the memory cell includes: applying zerovolts to all word line terminals not electrically connected to theselected cell; applying zero or positive volts to all buried wellterminals not electrically connected to the selected cell; and applyingzero volts to all substrate terminals not electrically connected to theselected cell.

In at least one embodiment, the array comprises a plurality of theasymmetric cells, each asymmetric cell comprising a gate; a word lineterminal electrically connected to the gate; a bit line terminal; asource line terminal; a buried layer; a buried well terminalelectrically connected to the buried layer; a substrate; and a substrateterminal electrically connected to the substrate, the method furthercomprising performing a bit-selective write logic-0 operation on thearray.

In at least one embodiment, the performance of a bit-selective writelogic-0 operation comprises: applying a positive voltage to the wordline terminal electrically connected to the selected cell; applying anegative voltage to the bit line terminal electrically connected to theselected cell; applying zero voltage to the source line terminalelectrically connected to the selected cell; applying a zero or positivebias to the buried well terminal electrically connected to the selectedcell; and applying zero bias to the substrate terminal electricallyconnected to the selected cell.

In at least one embodiment, the method includes: applying zero volts toall word line terminals not electrically connected to the selected cell;applying zero volts to all source line terminals not electricallyconnected to the selected cell; applying zero or positive volts to allburied well terminals not electrically connected to the selected cell;and applying zero volts to all substrate terminals not electricallyconnected to the selected cell.

In at least one embodiment, the bit line terminal is connected to thefloating body region via a Schottky contact.

In at least one embodiment, the bit line terminal is connected to thefloating body region via a Schottky contact, and wherein the performinga read operation comprises: applying a positive bias to the word lineterminal electrically connected to the selected cell; applying apositive bias to the bit line terminal electrically connected to theselected cell; applying zero bias to the source line terminalelectrically connected to the selected cell; applying a positive bias tothe buried well terminal electrically connected to the selected cell;and applying zero bias to the substrate terminal electrically connectedto the selected cell.

In at least one embodiment, the performance of a holding operationcomprises: applying a positive back bias to the buried well terminal;applying zero bias to the word line terminal; applying zero bias to thebit line terminal; leaving the source line terminal floating; andapplying zero bias to the substrate terminal.

In at least one embodiment, the source lines are each connected to onlya single one of the memory cells in the array.

In at least one embodiment, the performance of a write logic-1 operationcomprises: applying a negative voltage to the word line terminalelectrically connected to the selected cell; applying a positive voltageto the bit line terminal electrically connected to the selected cell;leaving floating the source line terminal connected to the selectedcell; applying a zero or positive bias to the buried well terminalelectrically connected to the selected cell; and applying zero bias tothe substrate terminal electrically connected to the selected cell.

In at least one embodiment, the performance of a write logic-0 operationcomprises: applying zero voltage to the word line terminal electricallyconnected to the selected cell; applying a negative voltage to the bitline terminal electrically connected to the selected cell; leavingfloating the source line terminal electrically connected to the selectedcell; applying a zero or positive bias to the buried well terminalelectrically connected to the selected cell; and applying zero bias tothe substrate terminal electrically connected to the selected cell.

In at least one embodiment, the performance of a bit-selective writelogic-0 operation comprises: applying a positive voltage to the wordline terminal electrically connected to the selected cell; applying anegative voltage to the bit line terminal electrically connected to theselected cell; leaving floating the source line terminal electricallyconnected to the selected cell; applying a zero or positive bias to theburied well terminal electrically connected to the selected cell; andapplying zero bias to the substrate terminal electrically connected tothe selected cell.

In at least one embodiment, the potential in the floating body of thememory cell designated state logic-1 is designated as V_(FB1), themethod further comprising: reducing write logic-0 disturb to unselectedmemory cells by applying the positive voltage to the word line terminalelectrically connected to the selected cell in an amount configured toincrease the floating body potential of the selected cell by V_(FB1)/2;applying the negative voltage to the bit line terminal electricallyconnected to the selected cell in an amount of about −V_(FB1)/2;applying either ground or a slightly positive voltage to the bit lineterminals of the array not connected to the selected cell; and applyinga negative voltage to the word line terminals not electrically connectedto the selected cell.

In another aspect of the present invention, a method of manufacturing amemory cell is provided including: growing a thin silicon oxide layer ona surface of a substrate; depositing a silicon nitride layer on thesilicon oxide layer; depositing a polysilicon layer over the siliconnitride layer; applying a pattern opening areas of the silicon oxidelayer, the silicon nitride layer and the polysilicon layer to be openedto form a trench; patterning the silicon oxide, silicon nitride andpolysilicon layers by lithography and then etching to create the trench;growing silicon oxide films in the trench to form an insulating layer ofthe memory cell; removing the silicon nitride layer and the polysiliconlayer; forming a buried layer region by ion implantation; forming asilicon oxide or high-dielectric material gate insulation layer on thesurface of the silicon oxide layer; depositing a polysilicon or metalgate layer on the gate insulation layer; forming a spacer region on bothsides of the gate 60; forming a source line region and a bit line regionin the silicon oxide layer by ion implantation, wherein gap regionsbetween the gate and the source line region, and between the gate andthe bit line region result from the forming a spacer region on bothsides.

In at least one embodiment, the method includes performing anotherlithography step to cover an area above a region between the bit lineregion, thereby maintaining one of the gap regions while eliminating theother of the gap regions; and forming an extension of the source lineregion where the gap was eliminated, using an ion implantation step.

In another aspect of the present invention, a method of manufacturing amemory cell is provided, including: growing a thin silicon oxide layeron a surface of a substrate; depositing a silicon nitride layer on thesilicon oxide layer; depositing a polysilicon layer over the siliconnitride layer; applying a pattern opening areas of the silicon oxidelayer, the silicon nitride layer and the polysilicon layer to be openedto form a trench; patterning the silicon oxide, silicon nitride andpolysilicon layers by lithography and then etching to create the trench;growing silicon oxide films in the trench to form an insulating layer ofthe memory cell; removing the silicon nitride layer and the polysiliconlayer; forming a buried layer region by ion implantation; forming asilicon oxide or high-dielectric material gate insulation layer on thesurface of the silicon oxide layer; depositing a polysilicon or metalgate layer on the gate insulation layer; forming a source line regionand a bit line region in the silicon oxide layer by ion implantation;performing a lithography step to block an area above the source lineregion, while leaving exposed an area above the bit line region to betransformed to a gap region between the gate and the bit line region,while blocking a remainder of the area above the bit line region; andchanging, by ion implantation, a conductivity type of the surface regionof the bit line region having been left exposed, thereby forming the gapregion.

In another aspect of the present invention, a method of manufacturing afloating body memory cell to improve a read signal thereof, is provided,including: providing a substrate; forming a buried well region in thesubstrate by ion implantation; growing a silicon oxide layer on asurface of the substrate; depositing a polysilicon layer on the siliconoxide layer; depositing a silicon nitride layer on the polysiliconlayer; opening an area to form a trench, using a lithography process;creating the trench by etching; forming a region at the bottom of thetrench by ion implantation; growing or depositing silicon oxide in thetrench to from an insulating layer of the memory cell; removing thesilicon nitride layer and the polysilicon layer; forming a gateinsulator on a surface of the silicon oxide; forming a gate over thegate insulator; and forming, by ion implantation, a source line regionof a first conductivity type and a bit line region of a secondconductivity type.

According to another aspect of the present invention, a semiconductormemory device having an electrically floating body with improved readoperation is provided. Methods of operation and manufacturing of thesemiconductor device are also provided. Applications of the memory cell,for example as configuration memory in a field programmable logic array(FPGA) or as a reference cell that can be used in comparing the state ofa floating body memory device are also provided.

These and other features of the invention will become apparent to thosepersons skilled in the art upon reading the details of the cells,arrays, devices and methods as more fully described below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B illustrate prior art floating body memory cells.

FIGS. 2A and 2B are schematic, cross-sectional illustrations of memorycells according to embodiments of the present invention.

FIG. 2C schematically illustrates an equivalent circuit representationof the memory cells shown in FIGS. 2A and 2B.

FIG. 2D schematically illustrates a bipolar device inherent in memorydevices of FIGS. 2A-2B.

FIG. 2E schematically illustrates multiple cells of the type in FIG. 2Aand/or FIG. 2B joined in an array to make a memory device.

FIG. 3A schematically illustrates performance of a holding operation ona memory array according to an embodiment of the present invention.

FIG. 3B illustrates bias conditions applied on the terminals of a memorycell of the array of FIG. 3A.

FIG. 4A shows an energy band diagram characterizing an intrinsic n-p-nbipolar device when a floating body region is positively charged and apositive bias voltage is applied to a buried well region of a memorycell according to an embodiment of the present invention.

FIG. 4B shows an energy band diagram of an intrinsic n-p-n bipolardevice when a floating body region is neutrally charged and a biasvoltage is applied to a buried well region of a memory cell according toan embodiment of the present invention.

FIG. 5A shows a graph of the net current I flowing into or out of afloating body region as a function of the potential V of the floatingbody, according to an embodiment of the present invention.

FIG. 5B shows a schematic curve of a potential energy surface (PES) of amemory cell according to an embodiment of the present invention.

FIG. 5C illustrates a charge stored in a floating body region of amemory cell as a function of a potential applied to a buried wellregion, connected to a BW terminal, according to an embodiment of thepresent invention.

FIG. 5D schematically illustrates performance of an alternative holdingoperation on a memory array employing an intrinsic silicon controlledrectifier principle according to an embodiment of the present invention.

FIG. 5E illustrates bias conditions applied on the terminals of a memorycell of the array of FIG. 5D.

FIG. 6A is a schematic view of a memory array showing exemplary biasconditions for performing a read operation on the memory array,according to an embodiment of the present invention.

FIG. 6B shows exemplary bias conditions applied to a selected memorycell during the read operation noted with regard to the array in FIG.6A.

FIG. 6C shows an exemplary sensing circuit connected to a selectedmemory cell during a read operation according to an embodiment of thepresent invention.

FIG. 7A shows an energy band diagram of an intrinsic bipolar device,according to an embodiment of the present invention, with a positivebias applied to the BL terminal and where the floating body region ispositively charged.

FIG. 7B shows an energy band diagram of the intrinsic bipolar device,according to an embodiment of the present invention, when the floatingbody region is neutrally charged.

FIG. 8A shows a drain current-gate voltage relationship of a memory cellwhen the floating body is positively charged and when the floating bodyis neutral, according to an embodiment of the present invention.

FIG. 8B shows a representative drain current-gate voltage relationshipwhen the current flow is fully controlled by the channel region (i.e. inthe absence of the gap region) of the memory cell, according to anembodiment of the present invention.

FIG. 9A is a schematic illustration of a memory cell array showingexemplary bias conditions for a write logic-1 operation on the memoryarray through an impact ionization mechanism, according to an embodimentof the present invention.

FIG. 9B illustrates bias conditions on an exemplary selected memory cellfrom the array of FIG. 9A.

FIG. 10A is a schematic illustration showing bias conditions for a writelogic-1 operation using band-to-band tunneling mechanism performed on amemory array according to an embodiment of the present invention.

FIG. 10B is a schematic view showing bias condition on an exemplaryselected memory cell in the embodiment of array shown in FIG. 10A.

FIG. 11A is a schematic illustration showing bias conditions for a writelogic-0 operation performed on a memory array according to an embodimentof the present invention.

FIG. 11B is a schematic illustration of bias conditions applied to anexemplary selected memory cell from the memory array of FIG. 11A.

FIG. 12A is a schematic illustration showing bias conditions applied fora bit-selective write logic-0 operation performed on a memory arrayaccording to an embodiment of the present invention.

FIG. 12B illustrates bias conditions applied to the terminals of anexemplary selected memory cell from the array of FIG. 12A.

FIG. 12C is a graph illustrating that V_(TS) is inversely dependent onthe potential difference between emitter and collector terminals(V_(CE)).

FIGS. 13A through 13G provide schematic illustrations at various stagesof an example of a manufacturing process to obtain a memory cellaccording to an embodiment of the present invention.

FIGS. 14A-14C schematically illustrate an alternative embodiment of someof the steps of the manufacturing process of FIGS. 13A-13G.

FIGS. 15A and 15B show cross sectional views of memory cell according toanother embodiment of the present invention, which incorporate Schottkycontact.

FIG. 15C schematically illustrates an equivalent circuit representationof a memory cell of an embodiment such as shown in FIG. 15A or 15B.

FIG. 15D schematically illustrates a bipolar device inherent in theembodiments of FIGS. 15A-15B.

FIG. 15E shows a cross-sectional view of memory cell according toanother embodiment of the present invention incorporating Schottkycontact, comprising a silicon-on-insulator (SOI) substrate.

FIG. 16A is a schematic illustration showing an exemplary memory arrayof memory cells arranged in rows and columns, according to an embodimentof the present invention.

FIG. 16B is a schematic illustration of another exemplary memory arrayconstructed from memory cells according to another embodiment of thepresent invention.

FIG. 17A is a schematic, cross-sectional illustration of a memory cellshowing bias conditions applied to perform a holding operation thereon,according to an embodiment of the present invention.

FIG. 17B is a schematic, cross-sectional illustration of a memory cellshowing bias conditions applied to perform an alternative holdingoperation thereon, according to an embodiment of the present invention.

FIGS. 18A and 18B schematically illustrate bias conditions applied tomemory arrays according to two different embodiments of the presentinvention, respectively, to perform a read operation on each.

FIG. 18C schematically illustrates bias conditions applied on anexemplary selected memory cell from the array in FIG. 18A as well asfrom the array in FIG. 18B.

FIGS. 19A and 19B schematically illustrate arrays according to twoembodiments of the present invention, and show bias conditions appliedthereto to perform a write logic-1 operation through an impactionization mechanism.

FIG. 19C schematically illustrates a cross-sectional view of a selectedcell and the bias conditions applied thereto for performing the writelogic-1 operation thereon in the memory array of FIG. 19A or FIG. 19B.

FIGS. 20A and 20B schematically illustrate arrays according to twoembodiments of the present invention, and show bias conditions appliedthereto to perform a write logic-1 operation using a band-to-bandtunneling mechanism.

FIG. 20C schematically illustrates a cross-sectional view of a selectedcell and the bias conditions applied thereto for performing the writelogic-1 operation thereon in the memory array of FIG. 20A or FIG. 20B.

FIG. 20D schematically illustrates an alternative set of bias conditionsfor performing a band-to-band tunneling write logic-1 operationaccording to an embodiment of the present invention.

FIGS. 21A-21B schematically illustrate arrays according to twoembodiments of the present invention, and show bias conditions appliedthereto to perform a write logic-0 operation.

FIG. 21C schematically illustrates a cross-sectional view of a selectedcell and the bias conditions applied thereto for performing the writelogic-0 operation thereon in the memory array of FIG. 21A or FIG. 21B.

FIGS. 22A-22B schematically illustrate arrays according to twoembodiments of the present invention, and show bias conditions appliedthereto to perform a bit-selective write logic-0 operation.

FIG. 22C schematically illustrates a cross-sectional view of a selectedcell and the bias conditions applied thereto for performing thebit-selective write logic-0 operation thereon in the memory array ofFIG. 22A or FIG. 22B.

FIG. 23A illustrates a schematic cross-sectional view of a memory cellaccording to another embodiment of the present invention.

FIG. 23B illustrates a schematic cross-sectional view of a memory cellaccording to another embodiment of the present invention.

FIG. 23C schematically illustrates an equivalent circuit representationof the memory cell of FIG. 23A or 23B.

FIG. 23D schematically illustrates a memory array comprising two rows ofmemory cells according to an embodiment of the present invention.

FIG. 24A schematically illustrates a cross-sectional view of a memorycell and shows exemplary bias conditions applied thereto for performinga holding operation thereon, according to an embodiment of presentinvention.

FIG. 24B schematically illustrates a cross-sectional view of a memorycell and shows exemplary bias conditions applied thereto for performingan alternative holding operation thereon, according to an embodiment ofpresent invention.

FIG. 25 is a schematic, cross-sectional illustration of a selectedmemory cell showing exemplary bias conditions that may be applied to theselected memory cell to perform a write logic-1 operation thereon,according to an embodiment of the present invention.

FIG. 26A is a schematic, cross-sectional illustration of a selectedmemory cell and exemplary bias conditions applied thereto to perform awrite logic-0 operation thereon, according to an embodiment of thepresent invention.

FIG. 26B is a schematic, cross-sectional illustration of a selectedmemory cell showing exemplary bias conditions that can be appliedthereto to perform a bit-selective write logic-0 operation thereon,according to an embodiment of the present invention.

FIG. 27 schematically illustrates use of a memory cell as a latch,according to an embodiment of the present invention.

FIG. 28 schematically illustrates an alternative arrangement of the useof a memory cell as a configuration memory to configure connectivity inan FPGA, according to an embodiment of the present invention.

FIG. 29A is a schematic, cross-sectional illustration of a memory cellwhich can be used as a reference cell in sensing the state of a floatingbody memory cell according to an embodiment of the present invention.

FIG. 29B is a schematic, cross-sectional illustration of a memory cellwhich can be used as a reference cell in sensing the state of a floatingbody memory cell according to another embodiment of the presentinvention.

FIG. 30A is a schematic illustration of a top view of a memory cellaccording to another embodiment of the present invention.

FIGS. 30B and 30C are schematic, cross-sectional illustrations of thecell of FIG. 30A taken along the I-I′ and II-II′ cut lines of FIG. 30A,respectively.

FIG. 31 illustrates an algorithm that can be employed to refresh thedata stored in floating body memory cells in parallel, according to anembodiment of the present invention.

FIG. 32A shows an implementation of the algorithm of FIG. 31 usingfeedback loop based on a single-stage operational amplifier (op-amp)according to an embodiment of the present invention.

FIG. 32B shows another implementation of the algorithm of FIG. 31through a CMOS inverter, comprising an NMOS transistor and a PMOStransistor according to an embodiment of the present invention.

FIG. 32C is a graph illustrating the input voltage-output voltagerelationship for the inverter of FIG. 32B.

FIG. 32D schematically illustrates another implementation of thealgorithm of FIG. 31 with a mixed-signal feedback loop according to anembodiment of the present invention.

FIG. 32E schematically illustrates operation of a mixed-signal feedbackloop according to an embodiment of the present invention.

FIG. 32F illustrates simplified waveforms associated with the circuitoperation described in FIG. 32E.

FIG. 33 is a schematic, cross-sectional illustration of memory cellfabricated on a silicon-on-insulator (SOI) substrate according to anembodiment of the present invention.

FIG. 34A is a schematic illustration of a top view of a memory cell,which provides an electrical connection to a floating body regionthrough a sense region, according to an embodiment of the presentinvention.

FIGS. 34B and 34C show cross-sectional views of the memory cell of FIG.34A along the I-I′ and II-II′ cut lines, respectively.

FIGS. 35A through 35C show alternative embodiments, according to thepresent invention, of a memory cell comprising a three-dimensionalmemory structure.

FIG. 36A through 36C show alternative embodiments, according to thepresent invention, of a memory cell having a fin structure.

FIGS. 37A through 37G show schematic, cross-sectional views of memorycells at various stages in a manufacturing process according to anembodiment of the present invention.

FIG. 38A illustrates a schematic, cross-sectional view of a memory cellaccording to another embodiment of the present invention.

FIG. 38B illustrates an equivalent circuit representation of the memorycell of FIG. 38A.

FIG. 38C illustrates another equivalent circuit representation of thememory cell of FIG. 38A.

FIG. 38D schematically illustrates a memory array comprising memorycells according to an embodiment of the present invention.

FIG. 39A schematically illustrates performance of a holding operation ona memory array according to an embodiment of the present invention.

FIG. 39B schematically illustrates a cross-sectional view of a memorycell and shows exemplary bias conditions applied thereto for performingan alternative holding operation thereon, according to an embodiment ofpresent invention.

FIG. 40A schematically illustrates bias conditions applied on a memoryarray according to an embodiment of the present invention to perform awrite logic-1 operation.

FIG. 40B is a schematic, cross-sectional illustration of a selectedmemory cell showing exemplary bias conditions that may be applied to theselected memory cell to perform a write logic-1 operation thereon,according to an embodiment of the present invention.

FIG. 41A schematically illustrates bias conditions applied on a memoryarray according to an embodiment of the present invention to perform awrite logic-0 operation.

FIG. 41B is a schematic, cross-sectional illustration of a selectedmemory cell showing exemplary bias conditions that may be applied to theselected memory cell to perform a write logic-0 operation thereon,according to an embodiment of the present invention.

FIGS. 42A and 42B show cross sectional views of a memory cell accordingto another embodiment of the present invention, which incorporateSchottky contact, which comprise intrinsic silicon controlled rectifier(SCR) device.

DETAILED DESCRIPTION OF THE INVENTION

Before the present devices, cells and methods are described, it is to beunderstood that this invention is not limited to particular embodimentsdescribed, as such may, of course, vary. It is also to be understoodthat the terminology used herein is for the purpose of describingparticular embodiments only, and is not intended to be limiting, sincethe scope of the present invention will be limited only by the appendedclaims.

Where a range of values is provided, it is understood that eachintervening value, to the tenth of the unit of the lower limit unlessthe context clearly dictates otherwise, between the upper and lowerlimits of that range is also specifically disclosed. Each smaller rangebetween any stated value or intervening value in a stated range and anyother stated or intervening value in that stated range is encompassedwithin the invention. The upper and lower limits of these smaller rangesmay independently be included or excluded in the range, and each rangewhere either, neither or both limits are included in the smaller rangesis also encompassed within the invention, subject to any specificallyexcluded limit in the stated range. Where the stated range includes oneor both of the limits, ranges excluding either or both of those includedlimits are also included in the invention.

Unless defined otherwise, all technical and scientific terms used hereinhave the same meaning as commonly understood by one of ordinary skill inthe art to which this invention belongs. Although any methods andmaterials similar or equivalent to those described herein can be used inthe practice or testing of the present invention, the preferred methodsand materials are now described. All publications mentioned herein areincorporated herein by reference to disclose and describe the methodsand/or materials in connection with which the publications are cited.

It must be noted that as used herein and in the appended claims, thesingular forms “a”, “an”, and “the” include plural referents unless thecontext clearly dictates otherwise. Thus, for example, reference to “acell” includes a plurality of such cells and reference to “the terminal”includes reference to one or more terminals and equivalents thereofknown to those skilled in the art, and so forth.

The publications discussed herein are provided solely for theirdisclosure prior to the filing date of the present application. Nothingherein is to be construed as an admission that the present invention isnot entitled to antedate such publication by virtue of prior invention.Further, the dates of publication provided may be different from theactual publication dates which may need to be independently confirmed.

Drawing figures in this specification, particularly diagramsillustrating semiconductor structures, are drawn to facilitateunderstanding through clarity of presentation and are not drawn toscale. In the semiconductor structures illustrated, there are twodifferent conductivity types: p-type where the majority charge carriersare positively charged holes that typically migrate along thesemiconductor valence band in the presence of an electric field, andn-type where the majority charge carriers are negatively chargedelectrons that typically migrate along the conduction band in thepresence of an electric field. Dopants may be introduced into anintrinsic semiconductor (where the quantity of holes and electrons areequal and the ability to conduct electric current is low: much betterthan in an insulator, but far worse than in a region doped to beconductive—hence the “semi-” in “semiconductor”) to create one of theconductivity types.

When dopant atoms capable of accepting another electron (known as“acceptors”) are introduced into the semiconductor lattice, the “hole”where an electron can be accepted becomes a positive charge carrier.When many such atoms are introduced, the conductivity type becomesp-type and the holes resulting from the electrons being “accepted” arethe majority charge carriers. Similarly, when dopant atoms capable ofdonating another electron (known as “donors”) are introduced into thesemiconductor lattice, the donated electron becomes a negative chargecarrier. When many such atoms are introduced, the conductivity typebecomes n-type and the “donated” electrons are the majority chargecarriers.

The quantities of dopant atoms used may vary widely over orders ofmagnitude of final concentration as a matter of design choice. Howeverit is the nature of the majority carriers and not their quantity thatdetermines if the material is p-type or n type. Sometimes in the art,heavily, medium, and lightly doped p-type material is designated p+, pand p− respectively while heavily, medium, and lightly doped n-typematerial is designated n+, n and n− respectively. Unfortunately, thereare no precise definitions of when a “+” or a “−” is an appropriatequalifier, so to avoid overcomplicating the disclosure the simpledesignations p type and n-type abbreviated “p” or “n” respectively areused without qualifiers throughout this disclosure. Persons of ordinaryskill in the art will appreciate that there are many considerations thatcontribute to the choice of doping levels in any particular embodimentas a matter of design choice.

Numerous different exemplary embodiments are presented. In many of themthere are common characteristics, features, modes of operation, etc.When like reference numbers are used in different drawing figures, theyare used to indicate analogous, similar or identical structures toenhance the understanding of the present invention by clarifying therelationships between the structures and embodiments presented in thevarious diagrams—particularly in relating analogous, similar oridentical functionality to different physical structures.

Definitions

The phrase “conductivity type” as used herein, refers to the type thetype of majority carriers present in a semiconductor region. In thesemiconductor structures illustrated, there are two differentconductivity types: p-type where the majority charge carriers arepositively charged holes that typically migrate along the semiconductorvalence band in the presence of an electric field, and n-type where themajority charge carriers are negatively charged electrons that typicallymigrate along the conduction band in the presence of an electric field.

The phrase “bi-stable memory cell” as used herein, refers to a memorycell having two stable states, which are separated by an energy barrier.

DETAILED DESCRIPTION

FIGS. 1A and 1B illustrate floating body memory cells 50P and 250P,respectively, for example, as described in U.S. Patent ApplicationPublication No. 2010/00246284 to Widjaja et al., titled “SemiconductorMemory Having Floating Body Transistor and Method of Operating”, “Scaled1T-Bulk Devices Built with CMOS 90 nm Technology for Low-Cost eDRAMApplications”, Ranica et al., 2005 Symposium on VLSI Technology, Digestof Technical Papers (“Ranica”) and U.S. Pat. No. 6,937,516“Semiconductor Device”, Fazan and Okhonin (“Fazan”), each of which arehereby incorporated herein, in their entireties, by reference thereto.FIGS. 1A and 1B illustrate floating body memory cells fabricated in abulk silicon substrate and on a silicon-on-insulator (SOI) substrate,respectively. In a floating body memory, the different memory states arerepresented by different levels of charge in the floating body 24.

In memory design in general, sensing and amplifying the state of amemory cell is an important aspect of the design. This is true as wellof floating body DRAM memories. Different aspects and approaches toperforming a read operation are known in the art, such as thosedisclosed in “A Design of a Capacitor-less 1T-DRAM Cell UsingGate-Induced Drain Leakage (GIDL) Current for Low-power and High-speedEmbedded Memory”, Yoshida et al., pp. 913-918, International ElectronDevices Meeting, 2003 (“Yoshida”) which is incorporated by referenceherein in its entirety; in U.S. Pat. No. 7,301,803 “Bipolar readingtechnique for a memory cell having an electrically floating bodytransistor” (“Okhonin-2”) which is incorporated by reference herein inits entirety; in “Memory Design Using One-Transistor Gain Cell on SOI”,T. Ohsawa et al., pp. 152-153, Tech. Digest, 2002 IEEE InternationalSolid-State Circuits Conference, February 2002) (“Ohsawa-1”); in “An18.5 ns 128 Mb SOI DRAM with a Floating Body Cell”, Ohsawa et al., pp.458-459, 609, IEEE International Solid-State Circuits Conference, 2005(“Ohsawa-2”); and in U.S. Patent Application Publication No.2009/0016101 titled “Reading technique for memory cell with electricallyfloating body transistor” (hereafter referred to as “Okhonin-3”), all ofwhich documents are hereby incorporated herein, in their entireties, byreference thereto. Both Yoshida and Okhonin-2 disclose a method ofgenerating a read current from a standard MOSFET floating body memorycell manufactured in SOI-CMOS processes. Okhonin-2 describes using theintrinsic BJT transistor inherent in the standard MOSFET structure togenerate the read current, while Okhonin-3 describes a spike readingtechnique applicable to a DRAM cell. Ohsawa-1 and Ohsawa-2 disclose adetailed sensing scheme for use with standard MOSFET floating bodymemory cells implemented in both SOI and standard bulk silicon whichcompares the current of a sensed cell to the average of two referencecells, one written to logic-0 state and the other written to logic-1state.

One method to sense the state of a floating body memory cell is throughmonitoring the cell current of the floating body memory cell. If thememory cell is in a logic-1 state having holes in the floating bodyregion 24, then the memory cell will have a lower threshold voltage(gate voltage where the transistor is turned on), and consequently ahigher cell current, compared to the floating body memory cell 50 is inLOGIC-0 state having no holes in the floating body region 24. However,the difference between the threshold voltage of memory cells in LOGIC-0and LOGIC-1 state decreases as the floating body memory cell 50 is beingscaled to smaller geometry due to the lower floating body capacitanceand/or higher gate oxide capacitance.

According to at least one embodiment of the present invention, asemiconductor memory device is provided with an electrically floatingbody with improved read operation. Methods of operation andmanufacturing of the semiconductor device are also provided.Applications of the memory cell, for example as configuration memory ina field programmable logic array (FPGA) or as a reference cell that canbe used in comparing the state of a floating body memory device, forexample, as described in Widjaja and Ranica, are also provided.

FIG. 2A illustrates a schematic cross-sectional view of a memory cell 50according to an embodiment of the present invention. Memory cell 50includes a substrate 12 of a first conductivity type such as p-type, forexample. Substrate 12 is typically made of silicon, but may also (oralternatively) comprise, for example, germanium, silicon germanium,gallium arsenide, carbon nanotubes, or other semiconductor materials. Insome embodiments of the invention, substrate 12 may be the bulk materialof the semiconductor wafer. In other embodiments, substrate 12 may be awell of the first conductivity type embedded in either a well of thesecond conductivity type or, alternatively, in the bulk of thesemiconductor wafer of the second conductivity type, such as n-type, forexample, (not shown in the figures). To simplify the description, thesubstrate 12 will usually be drawn as the semiconductor bulk material asit is in FIG. 2A.

A buried layer 22 of a second conductivity type such as n-type, forexample, is provided in the substrate 12. Buried layer 22 may be formedby an ion implantation process on the material of substrate 12.Alternatively, buried layer 22 can also be grown epitaxially on top ofsubstrate 12.

A floating body region 24 of the first conductivity type, such asp-type, for example, is bounded on top by bit line region 16, sourceline region 18, gap region 17 and insulating layer 62, on the sides byinsulating layers 26, and on the bottom by buried layer 22. Floatingbody 24 may be the portion of the original substrate 12 above buriedlayer 22 if buried layer 22 is implanted. Alternatively, floating body24 may be epitaxially grown. Depending on how buried layer 22 andfloating body 24 are formed, floating body 24 may have the same dopingas substrate 12 in some embodiments or a different doping, if desired inother embodiments.

Insulating layers 26 (like, for example, shallow trench isolation(STI)), may be made of silicon oxide, for example, though otherinsulating materials may be used. Insulating layers 26 insulate cell 50from neighboring cells 50 when multiple cells 50 are joined in an array80 (e.g., FIG. 2E) to make a memory device. The bottom of insulatinglayer 26 may reside inside the buried region 22 allowing buried region22 to be continuous as shown in FIG. 2A. Alternatively, the bottom ofinsulating layer 26 may reside below the buried region 22 as shown inthe cross-sectional view of another embodiment of memory cell 50 in FIG.2B. This requires a shallower insulating layer 28 (shown in dashed linesin FIG. 2B), which insulates the floating body region 24, but allows theburied layer 22 to be continuous in the perpendicular direction of thecross-sectional view shown in FIG. 2B. For simplicity, only memory cell50 with continuous buried region 22 in all directions will be shown fromhereon.

A bit line region 16 having a second conductivity type, such as n-type,for example, is provided in floating body region 24, so as to bound aportion of the top of the floating body region in a manner discussedabove, and is exposed at surface 14. Bit line region 16 may be formed byan implantation process on the material making up substrate 12,according to any implantation process known and typically used in theart. Alternatively, a solid state diffusion process could be used toform bit line region 16.

A source line region 18 having a second conductivity type, such asn-type, for example, is also provided in floating body region 24, so asto bound a portion of the top of the floating body region in a mannerdiscussed above, and is exposed at cell surface 14. Source line region18 may be formed by an implantation process on the material making upsubstrate 12, according to any implantation process known and typicallyused in the art. Alternatively, a solid state diffusion process could beused to form source line region 18.

A gate 60 is positioned in between the bit line region 16 and sourceline region 18 and above the floating body region 24. The gate 60 isinsulated from floating body region 24 by an insulating layer 62.Insulating layer 62 may be made of silicon oxide and/or other dielectricmaterials, including high-K dielectric materials, such as, but notlimited to, tantalum peroxide, titanium oxide, zirconium oxide, hafniumoxide, and/or aluminum oxide. The gate 60 may be made of, for example,polysilicon material or metal gate electrode, such as tungsten,tantalum, titanium and their nitrides.

Memory cell 50 is asymmetric in that there is a gap region 17 formednear the area of the bit line region 16. As a result, there is nooverlap between the area underneath the gate region 60, often referredto as the channel region 19, and the bit line region 16.

Cell 50 includes several terminals: word line (WL) terminal 70electrically connected to gate 60, bit line (BL) terminal 74electrically connected to bit line region 16, source line (SL) terminal72 electrically connected to source line region 18, buried well (BW)terminal 76 electrically connected to buried layer 22, and substrateterminal 78 electrically connected to the substrate 12.

FIG. 2C illustrates an equivalent circuit representation of memory cell50. Inherent in memory cell 50 are metal-oxide-semiconductor (MOS)transistor 20, formed by bit line region 16, gate 60, source line region18, and floating body region 24, and bipolar devices 30 a and 30 b,formed by buried well region 22, floating body region 24, and bit lineregion 16 or source line region 18, respectively.

Also inherent in memory device 50 is bipolar device 30 c, formed by bitline region 16, floating body 24, and source line region 18. Fordrawings clarity, bipolar device 30 c is shown separately in FIG. 2D.

FIG. 2E schematically illustrates an exemplary embodiment of a memoryarray 80 of memory cells 50 (four exemplary instances of memory cell 50being labeled as 50 a, 50 b, 50 c and 50 d) arranged in rows andcolumns. In many, but not all, of the figures where array 80 appears,representative memory cell 50 a will be representative of a “selected”memory cell 50 when the operation being described has one (or more insome embodiments) selected memory cells 50. In such figures,representative memory cell 50 b will be representative of an unselectedmemory cell 50 sharing the same row as selected representative memorycell 50 a, representative memory cell 50 c will be representative of anunselected memory cell 50 sharing the same column as selectedrepresentative memory cell 50 a, and representative memory cell 50 dwill be representative of a memory cell 50 sharing neither a row or acolumn with selected representative memory cell 50 a.

Present in FIG. 2E are word lines 70 a through 70 n, source lines 72 athrough 72 n, bit lines 74 a through 74 p, buried well terminals 76 athrough 76 n, and substrate terminal 78. Representation of thelines/terminal with letters a-n or a through p, includes not onlyembodiments which include literally twelve lines/terminals (i.e.,a,b,c,d,e,f,g,h,i,j,k,l,m,n,o,p) or fourteen lines/terminals (i.e.,a,b,c,d,e,f,g,h,i,j,k,l,m,n,o,p), but is meant to more genericallyrepresent a plurality of such line terminals, which can be less thantwelve (i.e., as low as one or greater than twelve, thirteen or fourteen(much greater than fourteen up to any positive integer practical).

Each of the source lines 72 a through 72 n is associated with a singlerow of memory cells 50 and is coupled to the source line region 18 ofeach memory cell 50 in that row. Each of the bit lines 74 a through 74 pis associated with a single column of memory cells 50 and is coupled tothe bit line region 16 of each memory cell 50 in that column.

Substrate 12 is present at all locations under array 80. Persons ofordinary skill in the art will appreciate that one or more substrateterminals 78 may be present in one or more locations. Such skilledpersons will also appreciate that although array 80 is shown in FIG. 2Eas a single continuous array, many other organizations and layouts arepossible. For example, word lines may be segmented or buffered, bitlines may be segmented or buffered, source lines may be segmented orbuffered, the array 80 may be broken into two or more sub-arrays,control circuits such as word decoders, column decoders, segmentationdevices, sense amplifiers, write amplifiers may be arrayed around array80 or inserted between sub-arrays of array 80. Thus the presentinvention is not limited to the exemplary embodiments, features, designoptions, etc., shown and described.

Several operations can be performed by memory cell 50 such as holding,read, write LOGIC-1 and write LOGIC-0 operations.

FIG. 3A schematically illustrates performance of a holding operation onmemory array 80, while FIG. 3B shows the bias applied on the terminalsof a memory cell 50 during the holding operation. The holding operationis performed by applying a positive back bias to the BW terminal 76,zero bias on the WL terminal 70, BL terminal 74, SL terminal 72, andsubstrate terminal 78. The positive back bias applied to the buriedlayer region 22 connected to the BW terminal 76 will maintain the stateof the memory cell 50 that it is connected to. The positive bias appliedto the BW terminal 76 needs to generate sufficient electric field totrigger impact ionization mechanism as will be described through theband diagram shown in FIGS. 4A and 4B. The impact ionization rate as afunction of the electric field is for example described in “Physics ofSemiconductor Devices”, Sze S. M. and Ng K. K., which is herebyincorporated herein, in its entirety, by reference thereto.

In one embodiment the bias conditions for the holding operation onmemory cell 50 is: 0 volts is applied to WL terminal 70, 0 volts isapplied to BL terminal 74, 0 volts is applied to SL terminal 72, apositive voltage, for example, +1.2 volts is applied to BW terminal 76,and 0 volts is applied to the substrate terminal 78. In otherembodiments, different voltages may be applied to the various terminalsof memory cell 50 and the exemplary voltages described are not limiting.

FIG. 4A shows an energy band diagram characterizing the intrinsic n-p-nbipolar device 30 a when the floating body region 24 is positivelycharged and a positive bias voltage is applied to the buried well region22. The energy band diagram of the intrinsic n-p-n bipolar device 30 bcan be constructed in a similar manner, with the source line region 18(connected to the SL terminal 72) in place of the bit line region 16(connected to the BL terminal 74). The dashed lines indicate the Fermilevels in the various regions of the n-p-n transistor 30 a. The Fermilevel is located in the band gap between the solid line 27 indicatingthe top of the valence band (the bottom of the band gap) and the solidline 29 indicating the bottom of the conduction band (the top of theband gap) as is well known in the art. If floating body 24 is positivelycharged, a state corresponding to LOGIC-1, the bipolar transistors 30 aand 30 b will be turned on as the positive charge in the floating bodyregion lowers the energy barrier of electron flow into the base region.Once injected into the floating body region 24, the electrons will beswept into the buried well region 22 (connected to BW terminal 76) dueto the positive bias applied to the buried well region 22. As a resultof the positive bias, the electrons are accelerated and createadditional hot carriers (hot hole and hot electron pairs) through animpact ionization mechanism. The resulting hot electrons flow into theBW terminal 76 while the resulting hot holes will subsequently flow intothe floating body region 24. This process restores the charge onfloating body 24 and will maintain the charge stored in the floatingbody region 24 which will keep the n-p-n bipolar transistors 30 a and 30b on for as long as a positive bias is applied to the buried well region22 through BW terminal 76.

If floating body 24 is neutrally charged (the voltage on floating body24 being equal to the voltage on grounded bit line region 16), a statecorresponding to LOGIC-0, no current will flow through the n-p-n bipolardevices 30 a and 30 b. The bipolar devices 30 a and 30 b will remain offand no impact ionization occurs. Consequently memory cells in theLOGIC-0 state will remain in the LOGIC-0 state.

FIG. 4B shows an energy band diagram of the intrinsic bipolar device 30a when the floating body region 24 is neutrally charged and a biasvoltage is applied to the buried well region 22. In this state theenergy level of the band gap bounded by solid lines 27A and 29A isdifferent in the various regions of n-p-n bipolar device 30 a. Becausethe potential of the floating body region 24 and the bit line region 16is equal, the Fermi levels are constant, resulting in an energy barrierbetween the bit line region 16 and the floating body region 24. Solidline 23 indicates, for reference purposes, the energy barrier betweenthe bit line region 16 and the floating body region 24. The energybarrier prevents electron flow from the bit line region 16 (connected toBL terminal 74) to the floating body region 24. Thus the n-p-n bipolardevice 30 a and 30 b will remain off.

In the holding operation described with regard to FIG. 3A, there is noindividually selected memory cell. Rather the holding operation will beperformed at all cells connected to the same buried well terminal 76.

FIG. 5A shows a graph of the net current I flowing into or out of thefloating body region 24 as a function of the potential V of the floatingbody 24 (not drawn to scale). A negative current indicates a net currentflowing into the floating body region 24, while a positive currentindicates a net current flowing out of the floating body region 24. Atlow floating body 24 potential, between 0V and V_(FB0) indicated in FIG.5A, the net current is flowing into the floating body region 24 as aresult of the p-n diode formed by the floating body region 24 and theburied well region 22 being reverse biased. If the value of the floatingbody 24 potential is between V_(FB0) and V_(TS), the current will switchdirection, resulting in a net current flowing out of the floating bodyregion 24. This is because of the p-n diode, formed by the floating bodyregion 24 and the buried well region 22, being forward biased as thefloating body region 24 becomes increasingly more positive. As a result,if the potential of the floating body region 24 is less than V_(TS),then at steady state the floating body region 24 will reach V_(FB0). Ifthe potential of the floating body region 24 is higher than V_(TS), thecurrent will switch direction, resulting in a net current flowing intothe floating body region 24. This is as a result of the base currentflowing into the floating body region 24 being greater than the p-ndiode leakage current. When the floating body 24 potential is higherthan V_(FB1), the net current will be out of the floating body region24. This is because the p-n diode leakage current is once again greaterthan the base current of the bipolar devices 30 a and 30 b.

The holding operation results in the floating body memory cell havingtwo stable states: the LOGIC-0 state and the LOGIC-1 state separated byan energy barrier, which are represented by V_(FB0), V_(FB1), andV_(TS), respectively. FIG. 5B shows a schematic curve of a potentialenergy surface (PES) of the memory cell 50, which shows anotherrepresentation of the two stable states resulting from applying a backbias to the BW terminal 76 (connected to the buried well region 22).

The values of the floating body 24 potential where the current changesdirection, i.e. V_(FB0), V_(FB1), and V_(TS), can be modulated by thepotential applied to the BW terminal 76. These values are alsotemperature dependent.

The holding/standby operation also results in a larger memory window byincreasing the amount of charge that can be stored in the floating body24. Without the holding/standby operation, the maximum potential thatcan be stored in the floating body 24 is limited to the flat bandvoltage V_(FB) as the junction leakage current to regions 16 and 18increases exponentially at floating body potential greater than V_(FB).However, by applying a positive voltage to substrate terminal 78, thebipolar action results in a hole current flowing into the floating body24, compensating for the junction leakage current between floating body24 and regions 16 and 18. As a result, the maximum charge V_(MC) storedin floating body 24 can be increased by applying a positive bias to thesubstrate terminal 78 as shown in FIG. 5C. The increase in the maximumcharge stored in the floating body 24 results in a larger memory window.

FIGS. 5D and 5E illustrate bias condition for an alternative holdingoperation applied on memory array 80 and on a selected memory cell 50 a,as described for example in US 2010/0034041, “Method of OperatingSemiconductor Memory Device with Floating Body Transistor Using SiliconControlled Rectifier Principle” (“Widjaja-2”), which is incorporated byreference herein in its entirety. The holding operation may also beperformed by applying the following bias conditions: zero voltage isapplied to WL terminal 70, SL terminal 72, and BL terminal 74, apositive voltage is applied to the substrate terminal 78, while the BWterminal 76 is left floating. Under these conditions, if memory cell 50is in memory/data state “1” with positive voltage in floating body 24,the intrinsic silicon controlled rectifier (SCR) device of memory cell50, formed by the substrate 12, the buried well region 22, the floatingbody region 24, and the bit line region 16 or the source line region 18,is turned on, thereby maintaining the state “1” data. Memory cells instate “0” will remain in blocking mode, since the voltage in floatingbody 24 is not substantially positive and therefore floating body 24does not turn on the SCR device. Accordingly, current does not flowthrough the SCR device and these cells maintain the state “0” data. Inthis way, an array of memory cells 50 may be refreshed by periodicallyapplying a positive voltage pulse through substrate terminal 78. Thosememory cells 50 that are commonly connected to substrate terminal 78 andwhich have a positive voltage in body region 24 will be refreshed with a“1” data state, while those memory cells 50 that are commonly connectedto the substrate terminal 78 and which do not have a positive voltage inbody region 24 will remain in blocking mode, since their SCR device willnot be turned on, and therefore memory state “0” will be maintained inthose cells. In this way, all memory cells 50 commonly connected to thesubstrate terminal will be maintained/refreshed to accurately hold theirdata states. This process occurs automatically, upon application ofvoltage to the substrate terminal 78, in a parallel, non-algorithmic,efficient process. In one particular non-limiting embodiment, a voltageof about 0.0 volts is applied to BL terminal 74, a voltage of about 0.0volts is applied to WL terminal 70, about 0.0 volts is applied to SLterminal 72, and about +1.2 volts is applied to terminal 78, while theBW terminal 76 is left floating. However, these voltage levels may vary,while maintaining the relative relationships therebetween.

The amount of charge stored in the floating body 24 can be sensed bymonitoring the cell current of the memory cell 50. If the memory cell isin a logic-1 state having holes in the floating body region 24, then thememory cell will have a lower threshold voltage (gate voltage where thetransistor is turned on), and consequently a higher cell current,compared to when the floating body memory cell 50 is in LOGIC-0 statehaving no holes in the floating body region 24.

FIG. 6A is a schematic view of a memory array 80 showing exemplary biasconditions for performing a read operation on the memory array 80,according to an embodiment of the present invention. FIG. 6B showsexemplary bias conditions applied to a selected memory cell 50 a duringthe read operation noted with regard to array 80 in FIG. 6A. The readoperation is performed by applying the following bias conditions: apositive bias to the WL terminal 70 a, a positive bias to the BLterminal 74 a, zero bias to the SL terminal 72 a, zero or positive biasto the BW terminal 76 a, and zero bias to the substrate terminal 78 a.All unselected WL terminals 70 b (not shown) to 70 n have zero voltsapplied, all unselected BL terminals 74 b through 74 p have zero voltsapplied, all unselected SL terminals 72 b (not shown) through 72 n havezero volts applied.

In one embodiment the bias conditions for the read operation for memorycell 50 are: +1.2 volts is applied to WL terminal 70, +0.4 volts isapplied to BL terminal 74, 0.0 volts is applied to SL terminal 72, +1.2volts is applied to BW terminal 76, and 0.0 volts is applied to thesubstrate terminal 78. In other embodiments, different voltages may beapplied to the various terminals of memory cell 50 and the exemplaryvoltages described are not limiting. The positive voltage applied to BLterminal 74 may be less than the positive voltage applied to WL terminal70, in which the difference in the threshold voltage of the memory cell50 is employed to represent the state of the memory cell 50. Thepositive voltage applied to BL terminal 74 may also be greater than orequal to the positive voltage applied to WL terminal 70 and may generatesufficiently high electric field to trigger the bipolar read mechanism.

A sensing circuit typically connected to BL terminal 74 can be used todetermine the data state of the memory cell 50. Any sensing scheme knownin the art can be used in conjunction with memory cell 50. For example,the sensing schemes disclosed in Ohsawa-1 and Ohsawa-2 are incorporatedby reference herein in its entirety.

FIG. 6C shows an example of a sensing circuit connected to the BLterminal 74 a, which senses the state of the selected memory cell 50 a.The cell current of the memory cell 50 a is compared with that of thereference cell 50Ref, which will be described subsequently. Thedifference between the cell current of the memory cell 50 a and thereference cell 50Ref is amplified by the latch 1200. The results of thesensing circuit will be reflected in nodes 1250 and 1252, where node1252 will be at Vdd if the memory cell 50 a is in LOGIC-1 state and willbe at GND if the memory cell 50 a is in LOGIC-0 state.

When no reading operation is performed, the precharge signal is high,which will turn off transistors 1202 and 1204 and turn on transistors1214 and 1216, bringing the BL terminals 74 a and 74Ref to GND. Thelatch 1200 is also turned off when no reading operation is performed byturning off transistors 1206 and 1208. During read operation, theprecharge signal switches to low, enabling transistors 1202 and 1204,while turning off the transistors 1214 and 1216. Subsequently, theselected WL terminal 70 a and BL terminal 74 a (through column select CStransistors 1210 and 1212) are enabled. If memory cell 50 a is inLOGIC-1 state, it will pull node 1250 to a lower voltage than node 1252and vice versa. The latch signal is subsequently turned on, amplifyingthe difference between nodes 1250 and 1252 to Vdd and GND through latch1200.

The difference between the threshold voltage of a memory cells in thelogic-0 state and a memory cell in the LOGIC-1 state decreases as thefloating body memory cell 50 is being scaled to smaller geometry due tothe lower floating body capacitance and/or higher gate oxidecapacitance. Consequently, it is becoming increasingly difficult todistinguish the memory cell current between memory cells in LOGIC-0 andLOGIC-1 state.

The presence of the gap region 17 in the memory cell 50 increases thecell current ratio between memory cells in LOGIC-0 state versus LOGIC-1state. The cell current flowing through the memory cell from the BLterminal 74 to SL terminal 72 is now governed by both the amount ofcarriers in the channel region 19 underneath the gate 60, and thepotential barrier in the gap region 17 between the channel region 19 andthe bit line region 16. Both the carrier density in the channel region19 and the potential barrier in the gap region 17 are a function of thefloating body potential 24.

The surface region of the memory cell 50 can be represented as twodevices in series: a metal-oxide-semiconductor (MOS) capacitor (formedby the gate electrode 60, the gate dielectrics 62, and the channelregion 19) and a bipolar transistor (formed by the channel region 19,the gap region 17, and the bit line region 16).

When a positive voltage is applied to the gate 60, holes will be forcedaway from the silicon surface, creating a depletion region in the regionunder the gate 60. When the gate voltage reaches the threshold voltage(voltage at which a switch from p-type to n-type occurs), an inversionregion is formed where the surface region appears to change in characterfrom p-type to n-type and the electron concentration at surface 19exceeds that of holes at surface 19. The threshold voltage is affectedby the potential of the floating body region 24, where a positivelycharged floating body 24 (e.g. for a memory cell 50 in LOGIC-1 state)will result in a lower threshold voltage than a neutral floating body 24(e.g. for a memory cell 50 in LOGIC-0 state). Because the thresholdvoltage depends on the floating body 24 potential, the number ofcarriers in the channel region 19 available for conduction consequentlyalso depends on the floating body 24 potential. Since the thresholdvoltage is lower when the floating body 24 is positively charged, thenumber of carriers at a given voltage applied to the gate 60 will alsobe higher compared to when the floating body region 24 is neutrallycharged.

Once an inversion region is formed in the region 19 under the gate 60,the electrons will need to travel across the gap region 17. FIG. 7Ashows an energy band diagram of the intrinsic bipolar device (formed bythe channel region 19, the gap region 17, and the bit line region 16),according to an embodiment of the present invention, with a positivebias applied to the BL terminal 74 (connected to the bit line region16). If floating body 24 is positively charged, a state corresponding toLOGIC-1, the bipolar transistor will be turned on as the positive chargein the floating body region lowers the energy barrier of electron flowinto the base region. This will result in electron flow from the channelregion 19 to the gap region 17 and subsequently to the bit line region16.

FIG. 7B shows an energy band diagram of the intrinsic bipolar device,according to an embodiment of the present invention, when the floatingbody region 24 is neutrally charged. In this state, an energy barrierbetween the channel region 19 and the gap region 17 exists, which isrepresented by the solid line 33. The energy barrier prevents electronflow from the channel region 19 to the gap region 17 and subsequently tothe bit line region 16. Thus the intrinsic n-p-n bipolar device will beturned off.

Both devices in series, i.e. the MOS capacitor (formed by the gate 60,gate dielectrics 62, and the channel region 19) and the intrinsicbipolar device (formed by the channel region 19, the gap region 17, andthe bit line region 16), are affected by the floating body 24 potentialin the same direction. A positively charged floating body 24 will resultin a lower threshold voltage of the MOS capacitor and a lower potentialbarrier between the channel region 19 and the drain junction 16 of theintrinsic bipolar device. Conversely, a neutrally charged floating body24 will result in both higher threshold voltage and higher potentialbarrier in the gap region 17. Consequently, the conductivity of theLOGIC-1 state of the memory cell 50 (i.e. positively charged floatingbody 24) is expected to be significantly higher than that of the LOGIC-0state (i.e. neutrally charged floating body 24).

The read mechanism may also be described by having the gap region 17being controlled by the fringing electric field from the gate 60, hencebeing only weakly controlled by the gate 60. As a result, the carrierflow through the gap region 17 is governed more dominantly by the energybarrier in the gap region 17, which is a function of the potential ofthe floating body 24.

FIG. 8A shows a drain current-gate voltage relationship of memory cell50 when the floating body 24 is positively charged and when the floatingbody 24 is neutral, according to an embodiment of the present invention.A combination of both high carrier density in the channel region 19 andlow potential barrier in the gap region 17 when the floating body 24 ispositively charged will result in significantly higher current flow(drain current from the BL terminal 74 to the SL terminal 72) comparedto when the floating body 24 is neutral. FIG. 8B shows a representativedrain current-gate voltage relationship when the current flow is fullycontrolled by the channel region (i.e. in the absence of the gap region17). With only one mechanism governing the current flow, the ratio ofthe cell current (i.e., drain current between states ‘1’ and ‘0’ issmaller, resulting in higher read error rate.

FIG. 9A is a schematic illustration of a memory cell array showingexemplary bias conditions for a write LOGIC-1 operation on the memoryarray 80 through an impact ionization mechanism, according to anembodiment of the present invention. FIG. 9B illustrates the biasconditions on an exemplary selected memory cell 50 a, according to theembodiment of FIG. 9A, where the following bias conditions are applied:a positive voltage is applied to the selected WL terminal 70, a positivevoltage is applied to the selected BL terminal 74, zero voltage isapplied to the selected SL terminal 72, zero or positive voltage isapplied to the selected BW terminal 76, and zero voltage is applied tothe substrate terminal 78. This positive voltage applied to the selectedBL terminal 74 a is greater than or equal to the positive voltageapplied to the selected WL terminal 70 a and may generate sufficientlyhigh enough electric field to trigger impact ionization mechanism.

In one particular non-limiting embodiment, about +1.2 volts is appliedto the selected WL terminal 70, about +1.2 volts is applied to theselected BL terminal 74, about 0.0 volts is applied to SL terminal 72,about 0.0 volts or +1.2 volts is applied to BW terminal 76, and about0.0 volts is applied to substrate terminal 78; while about 0.0 volts isapplied to the unselected WL terminals 70, unselected BL terminals 74,unselected SL terminals, and substrate terminal 78, and 0.0 volts or+1.2 volts is applied to BW terminal 76. These voltage levels areexemplary only and may vary from embodiment to embodiment. Thus theexemplary embodiments, features, bias levels, etc., described are notlimiting.

The positive bias applied to the selected BL terminal 74 will result ina depletion region formed around the bit line region 16, therebylowering the potential barrier in the gap region 17. This effect issometimes referred to as drain induced barrier lowering (DIBL). As aresult, carriers (e.g. electrons) will flow through the selected memorycell 50 a from the SL terminal 72 a to the BL terminal 74 a. Electronswill be accelerated in the pinch-off region (defined as the region nearthe surface 14 where the channel concentration is equal to the bulkdoping concentration) of the MOS device 20, creating hot carriers(electron and hole pairs) in the vicinity of the bit line region 16. Thegenerated holes will then flow into the floating body 24, putting thecell 50 a to the LOGIC-1 state.

Alternatively, a higher bias may be applied to the gate 60 (higher biasrelative to the bias applied to the gate 60 during the read operationdescribed above), to ensure that the channel region 19 underneath thegate 60 will be inverted regardless of the charge stored in the floatingbody region 24.

FIG. 10A is a schematic illustration showing bias conditions for a writeLOGIC-1 operation using band-to-band tunneling mechanism performed onmemory array 80 according to an embodiment of the present invention.FIG. 10B is a schematic view showing the bias condition on an exemplaryselected memory cell 50 a in the embodiment of array 80 shown in FIG.10A. A write LOGIC-1 operation using band-to-band tunneling mechanismcan be performed by applying the following bias conditions: a negativevoltage is applied to the selected WL terminal 70 a, a positive voltageis applied to the selected BL terminal 74 a, zero voltage is applied tothe selected SL terminal 72 a, zero or positive voltage is applied tothe selected BW terminal 76 a, and zero voltage is applied to thesubstrate terminal 78 a.

In one particular non-limiting embodiment, about −1.2 volts is appliedto the selected WL terminal 70 a, about +1.2 volts is applied to theselected BL terminal 74 a, about 0.0 volts is applied to SL terminal 72a, about 0.0 volts or +1.2 volts is applied to BW terminal 76 a, andabout 0.0 volts is applied to substrate terminal 78 a; while about 0.0volts is applied to the unselected WL terminals 70, unselected BLterminals 74, unselected SL terminals, and substrate terminal 78, and0.0 volts or +1.2 volts is applied to BW terminal 76. These voltagelevels are exemplary only may vary from embodiment to embodiment. Thusthe exemplary embodiments, features, bias levels, etc., described arenot limiting.

The negative charge on the gate 60 (connected to WL terminal 70) and thepositive voltage on bit line region 16 (connected to BL terminal 74)create a strong electric field (for example, about 10⁶ V/cm in silicon,as described in Sze, p. 104) between the bit line region 16 and thefloating body region 24 in the proximity of gate 60. This bends theenergy band sharply upward near the gate and bit line junction overlapregion, causing electrons to tunnel from the valence band to theconduction band, leaving holes in the valence band. The electrons whichtunnel across the energy band become the drain leakage current, whilethe holes are injected into floating body region 24 and become the holecharge that creates the LOGIC-1 state.

FIG. 11A is a schematic illustration showing bias conditions for a writeLOGIC-0 operation performed on memory array 80 according to anembodiment of the present invention. FIG. 11B is a schematicillustration of bias conditions applied to an exemplary selected memorycell 50 a from the memory array 80 of FIG. 11A. A write LOGIC-0operation can be performed by applying a negative voltage bias to theselected SL terminal 72 a, a zero voltage bias to the WL terminal 70 a,zero voltage bias to the BL terminal 74 a, zero or positive voltage biasto the BW terminal 76 a, and zero voltage bias to the substrate terminal78 a; while zero voltage is applied to the unselected SL terminals 72,zero voltage bias applied to the unselected WL terminals 70, zero orpositive bias applied to the BW terminal 76, and zero voltage biasapplied to the substrate 78. Under these conditions, the p-n junctionbetween floating body 24 and source line region 18 of the selected cell50 is forward-biased, evacuating holes from the floating body 24. Allmemory cells 50 sharing the same selected SL terminal 72 a will bewritten to simultaneously. To write arbitrary binary data to differentmemory cells 50, a write LOGIC-0 operation is first performed on all thememory cells to be written, followed by one or more write LOGIC-1operations on the memory cells that must be written to LOGIC-1.

In one particular non-limiting embodiment, about −1.2 volts is appliedto selected SL terminal 72 a, about 0.0 volts is applied to WL terminal70 a, about 0.0 volts is applied to BL terminal 74 a, about 0.0 volts or+1.2 volts is applied to BW terminal 76 a, and about 0.0 volts isapplied to substrate terminal 78 a, while zero voltage is applied to theunselected SL terminals 72, zero voltage bias applied to the unselectedWL terminals 70, zero or positive bias applied to the BW terminal 76,and zero voltage bias applied to the substrate 78. These voltage levelsare exemplary only may vary from embodiment to embodiment. Thus theexemplary embodiments, features, bias levels, etc., described are notlimiting.

FIG. 12A is a schematic illustration showing bias conditions applied fora bit-selective write LOGIC-0 operation performed on memory array 80according to an embodiment of the present invention. FIG. 12Billustrates bias conditions applied to the terminals of an exemplaryselected memory cell 50 a from the array 80 of FIG. 12A, which may beperformed by applying a positive voltage to the selected WL terminal 70a, a negative voltage to the selected BL terminal 74 a, zero voltagebias to the selected SL terminal 72 a, zero or positive voltage bias tothe BW terminal 76 a, and zero voltage to the substrate terminal 78;while zero voltage is applied to the unselected WL terminals 70, zerovoltage is applied to the unselected BL terminals 74, zero voltage biasis applied to the unselected SL terminals 72, zero or positive voltageis applied to the BW terminal 76, and zero voltage is applied to thesubstrate terminal 78. Under these conditions, the floating body 24potential will increase through capacitive coupling from the positivevoltage applied to the WL terminal 70. As a result of the floating body24 potential increase and the negative voltage applied to the BLterminal 74, the p-n junction between floating body region 24 and bitline region 16 is forward-biased, evacuating holes from the floatingbody 24.

To reduce undesired write LOGIC-0 disturb to other memory cells 50 in amemory array, the applied potential can be optimized as follows: if thefloating body 24 potential of state LOGIC-1 is referred to as V_(FB),then the voltage applied to the WL terminal 70 a is configured toincrease the floating body 24 potential by V_(FB1)/2 while −V_(FB1)/2 isapplied to BL terminal 74 a. Additionally, either ground or a slightlypositive voltage may also be applied to the BL terminals 74 ofunselected memory cells 50 that do not share the same BL terminal 74 aas the selected memory cell 50 a, while a negative voltage may also beapplied to the WL terminals 70 of unselected memory cells 50 that do notshare the same WL terminal 70 a as the selected memory cell 50 a.

As illustrated in FIGS. 12A and 12B, the following exemplary biasconditions may be applied to the selected memory cell 50 a to perform abit-selective write LOGIC-0 operation: a potential of about −0.2 voltsto the selected BL terminal 74 a, a potential of about +1.2 volts to theselected WL terminal 70 a, about 0.0 volts is applied to the selected SLterminal 72 a, a potential of about +1.2 volts to the BW terminal 76 a,about 0.0 volts to the substrate terminal 78 a.

The transition between LOGIC-0 state and LOGIC-1 state is defined byV_(TS) in FIGS. 5A and 5B. V_(TS) can be modulated by the potentialdifference across the emitter and collector terminals of the intrinsicn-p-n bipolar devices 30 a and 30 b, that is between the BW terminal 76and the BL terminal 74 and between the BW terminal 76 and SL terminal72, respectively. V_(TS) is inversely dependent on the potentialdifference between emitter and collector terminals (V_(CE)), as shown inFIG. 12C. The dependence of V_(TS) on V_(CE) may be utilized for thewrite LOGIC-0 operation. For example, the potential applied to the BWterminal 76 can be reduced during write LOGIC-0 operation, henceresulting in higher V_(TS), higher than the potential of the floatingbody region 24 V_(FB) of the selected memory cell 50 during writeLOGIC-0 operation. Because the V_(FB) is now less than V_(TS), a netcurrent flowing out of the floating body region 24 will be observed.

FIGS. 13A through 13G provide schematic illustrations at various stagesof an example of a manufacturing process to obtain memory cell 50according to an embodiment of the present invention. FIG. 13A isreferred to regarding early steps in the process. In an exemplary 130nanometer (nm) process, a thin silicon oxide layer 82 with a thicknessof about 100 A may be grown on the surface of substrate 12. This may befollowed by a deposition of about 200 A of polysilicon layer 84. This inturn may be followed by deposition of about 1200 A silicon nitride layer86. Other process geometries including, but not limited to: 250 nm, 180nm, 90 nm, 65 nm, etc., may be used. Similarly, other numbers of,thicknesses of, and combinations of protective layers 82, 84 and 86 maybe used. A pattern opening the areas to become trench 88 may be formedusing a lithography process. Then the silicon oxide 82, polysilicon 84,silicon nitride 86 layers may be subsequently patterned using thelithography process and then may be etched, followed by a silicon etchprocess, creating trench 88.

As shown in FIG. 13B, this may be followed by a silicon oxidation step,which will grow silicon oxide films in trench 88 which will becomeinsulating layer 26. In an exemplary 130 nm process, about 4000 Asilicon oxide may be grown. A chemical mechanical polishing step maythen be performed to polish the resulting silicon oxide films so thatthe silicon oxide layer 26 is flat relative to the silicon surface ofsubstrate 12. In other embodiments the top of insulating layer 26 mayhave different height relative to the silicon surface of substrate 12.The silicon nitride layer 86 and the polysilicon layer 84 may then beremoved which may then be followed by a wet etch process to removesilicon oxide layer 82 (and a portion of the silicon oxide films formedin the area of former trench 88). Other process geometries which mayinclude, but are not limited to 250 nm, 180 nm, 90 nm, 65 nm, etc., maybe used. Similarly, other insulating layer materials, heights, andthicknesses as well as alternate sequences of processing steps may beused.

As shown in FIG. 13C, an ion implantation step may then be performed toform the buried layer region 22 of a second conductivity (e.g. n-typeconductivity). The ion implantation energy may be optimized such thatthe bottom of the buried layer region 22 is formed deeper than thebottom of the insulating layer 26. Buried layer 22 isolates the eventualfloating body region 24 of the first conductivity type (e.g., p-type)from the substrate 12.

As shown in FIG. 13D, a silicon oxide or high-dielectric material gateinsulation layer 62 may then be formed on the silicon surface (e.g.about 100 A in an exemplary 130 nm process), which may then be followedby a polysilicon or metal gate 60 deposition (e.g. about 500 A in anexemplary 130 nm process). This is then followed by a lithography stepto define the gate, which is subsequently followed by an etching step.

The process sequence shown in FIGS. 13A through 13D describes theprocess flow typically employed in a standard complementary metal oxidesemiconductor (CMOS) process up to the gate formation step (with theexception of the ion implantation step performed to form the buriedlayer region 22 shown in FIG. 13C).

Subsequent to the gate formation step, a spacer region 64 may be formedon both sides of the gate 60, as shown in FIG. 13E. The spacer region 64is typically formed by a dielectric material deposition, such as siliconoxide, followed by a dry etching step.

FIG. 13F shows a subsequent ion implantation step of a secondconductivity (e.g. n-type implant) to form both the source line region18 and the bit line region 16. Because of the spacer region 64, there isa gap region 17S formed between the source line region and the channelregion 19, and a gap region 17 formed between the bit line region 16 andthe channel region 19.

FIG. 13G shows another lithography step which may be performed to coverthe area above the bit line region 16, but not the source line region18. An ion implantation of a second conductivity type (e.g. n-typeimplant) may then be performed to form an extension of the source lineregion 18 to the channel region 19, underneath the region previouslydefined by gap region 17S, thereby eliminating gap region 17S.

An alternative process is shown in FIGS. 14A through 14C, according toanother embodiment of the present invention. The cross-sectional view ofthe memory cell 50 shown in FIG. 14A follows the same process sequenceup to the gate formation step as shown in FIGS. 13A through 13D.

FIG. 14B shows a subsequent ion implantation step of a secondconductivity type (e.g. n-type implant) to form both the source lineregion 18 and the bit line region 16.

FIG. 14C shows the results of a subsequent lithography step which blocksthe area above the source line region 18, but leaves the area above thebit line region 16. A tilted ion implantation step of a firstconductivity type (e.g. p-type implant) is applied at an angle to thenormal direction of the surface 14 to change the conductivity type ofthe surface region of the bit line region 16 near the gate 60. As aresult, a gap region 17 is formed near the bit line region 16, as shownin FIG. 14C.

FIGS. 15A and 15B show cross sectional views of memory cell 150according to another embodiment of the present invention, whichincorporate Schottky contact. Memory cell 150 includes a substrate 12 ofa first conductivity type such as p-type, for example. Substrate 12 maybe typically made of silicon, but may also (or alternatively) comprise,for example, germanium, silicon germanium, gallium arsenide, carbonnanotubes, or other semiconductor materials. In some embodiments of theinvention, substrate 12 may be the bulk material of the semiconductorwafer. In other embodiments, substrate 12 may be a well of the firstconductivity type embedded in either a well of the second conductivitytype or, alternatively, in the bulk of the semiconductor wafer of thesecond conductivity type, such as n-type, for example, (not shown in thefigures). To simplify the description, the substrate 12 will usually bedrawn as the semiconductor bulk material as it is in FIG. 15A.

A buried layer 22 of a second conductivity type such as n-type, forexample, is provided in the substrate 12. Buried layer 22 may be formedby an ion implantation process on the material of substrate 12.Alternatively, buried layer 22 may be grown epitaxially on top ofsubstrate 12.

A floating body region 24 of the first conductivity type, such asp-type, for example, is bounded on top by surface 14, source line region18, and insulating layer 62 and gap region 17, on the sides byinsulating layers 26, and on the bottom by buried layer 22. Floatingbody 24 may be the portion of the original substrate 12 above buriedlayer 22 if buried layer 22 is implanted. Alternatively, floating body24 may be epitaxially grown. Depending on how buried layer 22 andfloating body 24 are formed, floating body 24 may have the same dopingas substrate 12 in some embodiments or a different doping, if desired inother embodiments.

Insulating layers 26 (which may comprise, for example, shallow trenchisolation (STI)), may be made of silicon oxide, for example, thoughother insulating materials may be used. Insulating layers 26 insulatecell 150 from neighboring cells 150 when multiple cells 150 are joinedin an array 180 to make a memory device. The bottom of insulating layer26 may reside inside the buried region 22 allowing buried region 22 tobe continuous as shown in FIG. 15A. Alternatively, the bottom ofinsulating layer 26 may reside below the buried region 22 as shown inthe embodiment of FIG. 15B. This requires a shallower insulating layer28, which insulates the floating body region 24, but allows the buriedlayer 22 to be continuous in the perpendicular direction of thecross-sectional view shown in FIG. 15B. For simplicity, only memory cell150 with continuous buried region 22 in all directions will be shownfrom hereon.

A source line region 18 having a second conductivity type, such asn-type, for example, is also provided in floating body region 24 and isexposed at surface 14. Source line region 18 may be formed by animplantation process formed on the material making up substrate 12,according to any implantation process known and typically used in theart. Alternatively, a solid state diffusion process could be used toform source line region 18.

The source line region 18 is electrically connected to source line (SL)terminal 72 through a conductive material 71. The conductive material 71may be made of, for example, polysilicon material, or metal electrode,such as tungsten, aluminum, and/or copper. The conductive material 71forms an ohmic contact 13 with the source line region 18.

The conductive material 73 forms a contact with the floating body region24. The conductive material 73 may be made of, for example, metalelectrode, such as tungsten or aluminum, or metal silicides, such asnickel silicide or platinum silicide. In contrast to the ohmic contactbetween conductive material 71 and source line region 18, conductivematerial 73 forms a Schottky contact 15 with the floating body region24.

A gate 60 is positioned in between the source line region 18 and theconductive material 73, and above the floating body region 24. The gate60 is insulated from floating body region 24 by an insulating layer 62.Insulating layer 62 may be made of silicon oxide and/or other dielectricmaterials, including high-K dielectric materials, such as, but notlimited to, tantalum peroxide, titanium oxide, zirconium oxide, hafniumoxide, and/or aluminum oxide. The gate 60 may be made of, for example,polysilicon material or metal gate electrode, such as tungsten,tantalum, titanium and/or their nitrides.

Cell 150 includes several terminals: word line (WL) terminal 70electrically connected to gate 60, bit line (BL) terminal 74electrically connected to conductive material 73, source line (SL)terminal 72 electrically connected to source line region 18 (through theconductive material 71), buried well (BW) terminal 76 electricallyconnected to buried layer 22, and substrate terminal 78 electricallyconnected to substrate 12.

FIG. 15C illustrates an equivalent circuit representation of memory cell150. Inherent in memory cell 150 are metal-oxide-semiconductor (MOS)transistor 120, formed by conductive material 73, gate 60, source lineregion 18, and floating body region 24; n-p-n bipolar device 130 a,formed by buried well region 22, floating body region 24, and conductivematerial 73, and n-p-n bipolar device 130 b, formed by buried wellregion 22, floating body region 24, and source line region 18,respectively.

Also inherent in memory device 150 is bipolar device 130 c, formed byconductive region 73, floating body 24, and source line region 18. Fordrawings clarity, bipolar device 130 c is shown separately in FIG. 15D.

Memory cell 150 may alternatively be fabricated on asilicon-on-insulator (SOI) substrate as illustrated in FIG. 15E, where aburied insulator layer 22, such as a buried oxide layer, bounds thefloating body region 24 at the bottom.

FIG. 16A is a schematic illustration showing an exemplary memory array180 of memory cells 150 (four exemplary instances of memory cell 150being labeled as 150 a, 150 b, 150 c and 150 d) arranged in rows andcolumns, according to an embodiment of the present invention. In many,but not all, of the figures where exemplary array 180 appears,representative memory cell 150 a will be representative of a “selected”memory cell 150 when the operation being described has one (or more insome embodiments) selected memory cells 150. In such figures,representative memory cell 150 b will be representative of an unselectedmemory cell 150 sharing the same row as selected representative memorycell 150 a, representative memory cell 150 c will be representative ofan unselected memory cell 150 sharing the same column as selectedrepresentative memory cell 150 a, and representative memory cell 150 dwill be representative of a memory cell 150 sharing neither a row or acolumn with selected representative memory cell 150 a.

Present in FIG. 16A are word lines 70 a through 70 n, source lines 72 athrough 72 n, bit lines 74 a through 74 p, buried well terminals 76 athrough 76 n, and substrate terminal 78. Each of the source lines 72 athrough 72 n is associated with a single row of memory cells 150 and iscoupled to the source line region 18 (via conductive material 71) ofeach memory cell 150 in that row. Each of the bit lines 74 a through 74p is associated with a single column of memory cells 150 and is coupledto the conductive material 73 of each memory cell 150 in that column.

Substrate 12 is present at all locations under array 180. Persons ofordinary skill in the art will appreciate that one or more substrateterminals 78 may be present in one or more locations. Such skilledpersons will also appreciate that while exemplary array 180 is shown asa single continuous array in FIG. 16A, that many other organizations andlayouts are possible such as, but not limited to: word lines may besegmented or buffered, bit lines may be segmented or buffered, sourcelines may be segmented or buffered, the array 180 may be broken into twoor more sub-arrays, control circuits such as word decoders, columndecoders, segmentation devices, sense amplifiers, write amplifiers maybe arrayed around exemplary array 180 or inserted between sub-arrays ofarray 180, etc. Thus the exemplary embodiments, features, designoptions, etc., described are not limiting.

FIG. 16B is a schematic illustration of another exemplary memory array182 constructed from memory cells 150, according to another embodimentof the present invention. In the memory array 182, the source lines 72 athrough 72 p (connected to the source line region 18 (via conductivematerial 71) are now each associated with a single column of memorycells 150, respectively. Each of the bit lines 74 a through 74 p is alsoassociated with a single column of memory cells 150 and is coupled tothe conductive material 73 of each memory cell 150 in that column.

Several operations can be performed on memory cell 150, includingholding, read, write LOGIC-1 and write LOGIC-0 operations.

FIG. 17A is a schematic, cross-section illustration of cell 150 showingbias conditions applied to perform a holding operation on memory cell150. The holding operation on memory cell 150 follows the same principleas the holding cell operation on memory cell 50 and is performed byapplying a positive back bias to the BW terminal 76, zero bias to the WLterminal 70, BL terminal 74, SL terminal 72, and substrate terminal 78.The positive back bias applied to the buried layer region 22 connectedto the BW terminal 76 will maintain the state of the memory cell 150that it is connected to. The holding operation can be performed in thesame manner when memory cells 150 are connected in memory arrayconfiguration 180 or 182. In the holding operation described in FIG. 17,there is no individually selected memory cell. Rather the holdingoperation will be performed on all cells connected to the same buriedwell terminal 76. The positive bias applied to the BW terminal 76 needsto generate a sufficient electric field to trigger an impact ionizationmechanism as will be described with reference to the band diagram shownin FIGS. 4A and 4B. The impact ionization rate as a function of theelectric field is for example described in Sze on pp. 37-41.

In one non-limiting embodiment, the bias conditions for the holdingoperation on memory cell 150 are: about 0.0 volts are applied to WLterminal 70, SL terminal 72, BL terminal 74, and substrate terminal 78,while about +1.2 volts are applied to the BW terminal 76. In otherembodiments, different voltages may be applied to various terminals ofmemory cells 150.

FIG. 17B illustrates bias conditions for an alternative holdingoperation applied on a memory cell 150, as described in Widjaja-2. Theholding operation may alternatively be performed by applying thefollowing bias conditions: zero voltage is applied to WL terminal 70, SLterminal 72, and BL terminal 74, a positive voltage is applied to thesubstrate terminal 78, while the BW terminal 76 is left floating. Underthese conditions, if memory cell 150 is in memory/data state “1” withpositive voltage in floating body 24, the intrinsic silicon controlledrectifier (SCR) device of memory cell 150, formed by the substrate 12,the buried well region 22, the floating body region 24, and the sourceline region 18 or the conductive material 73 forming Schottky contact 15with the floating body region 24, is turned on, thereby maintaining thestate “1” data. Memory cells in state “0” will remain in blocking mode,since the voltage in floating body 24 is not substantially positive andtherefore floating body 24 does not turn on the SCR device. Accordingly,current does not flow through the SCR device and these cells maintainthe state “0” data. In this way, an array of memory cells 150 may berefreshed by periodically applying a positive voltage pulse throughsubstrate terminal 78. Those memory cells 150 that are commonlyconnected to substrate terminal 78 and which have a positive voltage inbody region 24 will be refreshed with a “1” data state, while thosememory cells 150 that are commonly connected to the substrate terminal78 and which do not have a positive voltage in body region 24 willremain in blocking mode, since their SCR device will not be turned on,and therefore memory state “0” will be maintained in those cells. Inthis way, all memory cells 150 commonly connected to the substrateterminal will be maintained/refreshed to accurately hold their datastates. This process occurs automatically, upon application of voltageto the substrate terminal 78, in a parallel, non-algorithmic, efficientprocess. In one particular non-limiting embodiment, a voltage of about0.0 volts is applied to BL terminal 74, a voltage of about 0.0 volts isapplied to WL terminal 70, about 0.0 volts is applied to SL terminal 72,and about +1.2 volts is applied to terminal 78, while the BW terminal 76is left floating. However, these voltage levels may vary, whilemaintaining the relative relationships therebetween.

FIGS. 18A and 18B schematically illustrate bias conditions applied tothe memory array 180 and 182, respectively, to perform a read operationon each. FIG. 18C schematically illustrates bias conditions applied onan exemplary selected memory cell 150 a from array 180 in FIG. 18A aswell as from array 182 in FIG. 18B. Any sensing scheme known in the artcan be used with memory cell 150, including, for example, the sensingschemes disclosed by Ohsawa-1 and Ohsawa-2, which are each incorporatedby reference herein in their entireties.

Similar to the gap region 17 in the memory cell 50, the gap region 17 inmemory cell 150 (formed between the channel region 19 underneath thegate electrode 60 and the Schottky contact 15) increases the cellcurrent ratio between memory cells in LOGIC-0 and LOGIC-1 state. Thecell current flowing through the memory cell from the BL terminal 74 toSL terminal 72 is governed by both the amount of carriers in the channelregion 19 underneath the gate 60, and the potential barrier in the gapregion 17 between the channel region 19 and the bit line region 16. Boththe carrier density in the channel region 19 and the potential barrierin the gap region 17 are a function of the floating body potential 24.

The surface region of the memory cell 150 may be represented as twodevices in series: a metal-oxide-semiconductor (MOS) capacitor (formedby the gate electrode 60, the gate dielectrics 62, and the channelregion 19) and a bipolar transistor (formed by the channel region 19,the gap region 17, and the Schottky contact 15).

When a positive voltage is applied to the gate 60, holes will be forcedaway from the silicon surface, creating a depletion region in the regionin the region 19 under the gate 60. When the gate voltage reaches thethreshold voltage, an inversion region is formed where the surfaceregion appears to change in character from p-type to n-type and theelectron concentration at the surface exceeds that of holes. Thethreshold voltage is affected by the potential of the floating bodyregion 24, where a positively charged floating body 24 (e.g. for amemory cell 150 in LOGIC-1 state) will result in a lower thresholdvoltage than a neutral floating body 24 (e.g. for a memory cell 150 inLOGIC-0 state). Because the threshold voltage depends on the floatingbody 24 potential, the number of carriers in the channel region 19available for conduction consequently also depends on the floating body24 potential. Since the threshold voltage is lower when the floatingbody 24 is positively charged, the number of carriers at a given voltageapplied to the gate 60 will also be higher compared to when the floatingbody region 24 is neutrally charged.

Once an inversion region is formed in the region 19 under the gate 60,the electrons will need to travel across the gap region 17. If floatingbody 24 is positively charged, a state corresponding to LOGIC-1, thebipolar transistor will be turned on as the positive charge in thefloating body region lowers the energy barrier of electron flow into thebase region. This will result in electron flow from the channel region19 to the gap region 17 and subsequently to the conductive electrode 73.If the floating body 24 is neutrally charged, an energy barrier betweenthe channel region 19 and the gap region 17 exists. Thus, electron flowfrom the channel region 19 to the conductive electrode 73 through thegap region 17 will be prevented.

Both devices in series, i.e. the MOS capacitor (formed by the gate 60,gate dielectrics 62, and the channel region 19) and the intrinsicbipolar device (formed by the channel region 19, the gap region 17, andthe Schottky contact 15), are affected by the floating body 24 potentialin the same direction. A positively charged floating body 24 will resultin a lower threshold voltage of the MOS capacitor and a lower potentialbarrier between the channel region 19 and the Schottky contact 15 of theintrinsic bipolar device. Conversely, a neutrally charged floating body24 will result in both higher threshold voltage and higher potentialbarrier in the gap region 17. Consequently, the conductivity of theLOGIC-1 state of the memory cell 150 (i.e. positively charged floatingbody 24) is expected to be significantly higher than that of the LOGIC-0state (i.e. neutrally charged floating body 24).

The read mechanism may also be described by having the gap region 17being controlled by the fringing electric field from the gate 60, hencebeing only weakly controlled by the gate 60. As a result, the carrierflow through the gap region 17 is governed more dominantly by the energybarrier in the gap region 17, which is a function of the potential ofthe floating body 24.

In one embodiment the bias conditions for a read operation on memorycell 150 is: +1.2 volts is applied to WL terminal 70, +0.4 volts isapplied to BL terminal 74, 0 volts is applied to SL terminal 72, +1.2volts is applied to BW terminal 76, and 0 volts is applied to thesubstrate terminal 78. In other embodiments, different voltages may beapplied to the various terminals of memory cell 150. For example,because of the high resistivity of the Schottky contact 15 (compared toOhmic contact 13), a higher bias may be applied to the BL terminal 74 toincrease the current flow through the memory cell 150. The positivevoltage applied to BL terminal 74 may be less than the positive voltageapplied to WL terminal 70, in which the difference in the thresholdvoltage of the memory cell 50 is employed to represent the state of thememory cell 50. The positive voltage applied to BL terminal 74 may alsobe greater than or equal to the positive voltage applied to WL terminal70 and may generate sufficiently high electric field to trigger thebipolar read mechanism.

FIGS. 19A and 19B schematically illustrate arrays 180 and 182,respectively and show bias conditions applied thereto to perform a writeLOGIC-1 operation through impact ionization mechanism. FIG. 19Cschematically illustrates a cross-sectional view of a selected cell 150a and the bias conditions applied thereto for performing the writeLOGIC-1 operation thereon in memory array 180 or 182 from FIGS. 19A and19B, respectively. The following bias conditions are applied: a positivevoltage is applied to the selected WL terminal 70 a, a positive voltageis applied to the selected BL terminal 74 a, zero voltage is applied tothe selected SL terminal 72 a, zero or positive voltage is applied tothe selected BW terminal 76 a, and zero voltage is applied to thesubstrate terminal 78 a. The positive voltage applied to the selected BLterminal 74 a is greater than or equal to the positive voltage appliedto the selected WL terminal 70 a and may generate sufficiently highenough electric field to trigger impact ionization mechanism.

In one particular non-limiting embodiment, about +1.2 volts is appliedto the selected WL terminal 70 a, about +1.2 volts is applied to theselected BL terminal 74 a, about 0.0 volts is applied to SL terminal 72a, about 0.0 volts or +1.2 volts is applied to BW terminal 76 a, andabout 0.0 volts is applied to substrate terminal 78 a; while about 0.0volts is applied to the unselected WL terminals 70, unselected BLterminals 74, unselected SL terminals, and substrate terminal 78, and0.0 volts or +1.2 volts is applied to BW terminal 76. These voltagelevels are exemplary only and may vary from embodiment to embodiment.

The positive bias applied to the selected BL terminal 74 a will resultin a depletion region formed around the Schottky contact 15, therebylowering the potential barrier in the gap region 17. This effect issometimes referred to as drain induced barrier lowering (DIBL). As aresult, carriers (e.g. electrons) will flow through the selected memorycell 150 a from the SL terminal 72 a to the BL terminal 74 a. Electronswill be accelerated in the pinch-off region of the MOS device 120,creating hot carriers (electron and hole pairs) in the vicinity of theSchottky contact 15. The generated holes will then flow into thefloating body 24, putting the cell 150 a to the LOGIC-1 state.

Alternatively, a higher bias may be applied to the gate 60 (higher biasrelative to the bias applied to the gate 60 during the read operationdescribed above), to ensure that the channel region 19 underneath thegate 60 will be inverted regardless of the charge stored in the floatingbody region 24.

FIGS. 20A and 20B schematically illustrate memory arrays 180 and 182,respectively, and show bias conditions applied thereto to perform awrite LOGIC-1 operation using band-to-band tunneling mechanism,respectively. FIG. 20C schematically illustrates a cross-sectional viewof a selected cell 150 a and the bias conditions applied thereto toperform the write LOGIC-1 operation using band-to-band tunnelingmechanism thereon in memory array 180 or 182 from FIGS. 20A and 20B,respectively. The bias conditions applied in this example are: anegative voltage is applied to the selected WL terminal 70 a, a positivevoltage is applied to the selected BL terminal 74 a, zero voltage isapplied to the selected SL terminal 72 a, zero or positive voltage isapplied to the selected BW terminal 76 a, and zero voltage is applied tothe substrate terminal 78 a.

In one particular non-limiting embodiment, about −1.2 volts is appliedto the selected WL terminal 70 a, about +1.2 volts is applied to theselected BL terminal 74 a, about 0.0 volts is applied to SL terminal 72a, about 0.0 volts or +1.2 volts is applied to BW terminal 76 a, andabout 0.0 volts is applied to substrate terminal 78 a; while about 0.0volts is applied to the unselected WL terminals 70, unselected BLterminals 74, unselected SL terminals, and substrate terminal 78, and0.0 volts or +1.2 volts is applied to BW terminal 76. These voltagelevels are exemplary only and may vary from embodiment to embodiment.

The negative charge on the gate 60 (connected to WL terminal 70 a) andthe positive voltage on Schottky contact 15 (connected to BL terminal 74a) create a strong electric field between the Schottky contact 15 andthe floating body region 24 in the proximity of gate 60 (in the vicinityof the gap region 17). This bends the energy band sharply upward in thesurface area near the gate 60 and the Schottky contact 15 (in thevicinity of gap region 17), causing electrons to tunnel from the valenceband to the conduction band, leaving holes in the valence band. Theelectrons which tunnel across the energy band become the drain leakagecurrent, while the holes are injected into floating body region 24 andbecome the hole charge that creates the LOGIC-1 state.

The presence of the gap region 17 may reduce the effectiveness of theband-to-band tunneling mechanism since it decreases the overlap of thesurface area near the gate 60 and the Schottky contact 15.Alternatively, in memory array 182, the band-to-band tunneling writeLOGIC-1 operation can be performed by applying a positive bias to the SLterminal 72 a, zero voltage to the BL terminal 74 a, negative voltage tothe WL terminal 70 a, zero or positive bias to the BW terminal 76, andzero bias to the substrate terminal 78. This is illustrated in FIG. 20D.

FIGS. 21A and 21B schematically illustrate memory arrays 180 and 182,respectively, and show bias conditions applied thereto to perform awrite LOGIC-0 operation thereon. FIG. 21C schematically illustrates across-sectional view of a selected cell 150 a and the bias conditionsapplied thereto to perform the write logic-0 operation thereon foreither of arrays 180, 182. The write LOGIC-0 operation can be performedby applying a negative voltage bias to the selected SL terminal 72 a, azero voltage bias to the WL terminal 70, zero voltage bias to the BLterminal 74, zero or positive voltage bias to the BW terminal 76 (or 76a), and zero voltage bias to the substrate terminal 78 (or 78 a); whilezero voltage is applied to the unselected SL terminals 72, zero voltagebias applied to the unselected WL terminals 70, zero or positive biasapplied to the BW terminal 76, and zero voltage bias applied to thesubstrate terminal 78. Under these conditions, the p-n junction betweenfloating body 24 and source line region 18 of the selected cell 150 isforward-biased, evacuating holes from the floating body 24. All memorycells 150 sharing the same SL terminal 72 a will be written tosimultaneously. To write arbitrary binary data to different memory cells150, a write LOGIC-0 operation is first performed on all the memorycells to be written, followed by one or more write LOGIC-1 operations onthe memory cells that must be written to LOGIC-1.

In one particular non-limiting embodiment, about −1.2 volts is appliedto selected SL terminal 72 a, about 0.0 volts is applied to WL terminal70 a, about 0.0 volts or +1.2 volts is applied to BW terminal 76 or 76a, and about 0.0 volts is applied to substrate terminal 78 or 78 a.These voltage levels are exemplary only may vary from embodiment toembodiment.

FIGS. 22A and 22B schematically illustrate memory arrays 180 and 182,respectively, and show bias conditions applied thereto to perform abit-selective write LOGIC-0 operation thereon. FIG. 22C schematicallyillustrates a cross-sectional view of a selected cell 150 a and the biasconditions applied thereto to perform the bit-selective write LOGIC-0operation thereon for either of arrays 180, 182. The bias conditionsinclude applying a positive voltage to the selected WL terminal 70 a, anegative voltage to the selected BL terminal 74 a, zero voltage bias tothe selected SL terminal 72 a, zero or positive voltage bias to the BWterminal 76 or 76 a, and zero voltage to the substrate terminal 78 or 78a; while zero voltage is applied to the unselected WL terminals 70, zerovoltage is applied to the unselected BL terminals 74, zero voltage biasis applied to the unselected SL terminals 72, zero or positive voltageis applied to the BW terminal 76, and zero voltage is applied to thesubstrate terminal 78. Under these conditions, the floating body 24potential will increase through capacitive coupling from the positivevoltage applied to the WL terminal 70 a. As a result of the floatingbody 24 potential increase and the negative voltage applied to the BLterminal 74 a, the p-n junction between floating body region 24 and bitline region 16 is forward-biased, evacuating any holes from the floatingbody 24.

To reduce undesired write LOGIC-0 disturb to other memory cells 150 in amemory array, the applied potential can be optimized as follows: if thefloating body 24 potential of state LOGIC-1 is referred to as V_(FB1),then the voltage applied to the WL terminal 70 a is configured toincrease the floating body 24 potential by V_(FB1)/2 while −V_(FB1)/2 isapplied to BL terminal 74 a. Additionally, either ground or a slightlypositive voltage may also be applied to the BL terminals 74 ofunselected memory cells 150 that do not share the same BL terminal 74 aas the selected memory cell 150 a, while a negative voltage may also beapplied to the WL terminals 70 of unselected memory cells 150 that donot share the same WL terminal 70 a as the selected memory cell 150 a.

As illustrated in FIGS. 22A through 22C, the following exemplary biasconditions may be applied to the selected memory cell 150 a to perform abit-selective write LOGIC-0 operation: a potential of about −0.2 voltsto the selected BL terminal 74 a, a potential of about +1.2 volts to theselected WL terminal 70 a, about 0.0 volts is applied to the selected SLterminal 72 a, a potential of about +1.2 volts to the BW terminal 76 or76 a, about 0.0 volts to the substrate terminal 78 or 78 a.

FIG. 23A illustrates a schematic cross-sectional view of memory cell 250according to another embodiment of the present invention. Memory cell250 includes a substrate 12 of a first conductivity type such as p-type,for example. Substrate 12 is typically made of silicon, but may also (oralternatively) comprise, for example, germanium, silicon germanium,gallium arsenide, carbon nanotubes, or other semiconductor materials. Insome embodiments of the invention, substrate 12 may be the bulk materialof the semiconductor wafer. In other embodiments, substrate 12 may be awell of the first conductivity type embedded in either a well of thesecond conductivity type or, alternatively, in the bulk of thesemiconductor wafer of the second conductivity type, such as n-type, forexample, (not shown in the figures). To simplify the description, thesubstrate 12 will usually be drawn as the semiconductor bulk material asit is in FIG. 23A.

A buried layer 22 of a second conductivity type such as n-type, forexample, is provided in the substrate 12. Buried layer 22 may be formedby an ion implantation process on the material of substrate 12.Alternatively, buried layer 22 may be grown epitaxially on top ofsubstrate 12.

A floating body region 24 of the first conductivity type, such asp-type, for example, is bounded on top by bit line region 16, sourceline region 18, and insulating layer 62, on the sides by insulatinglayers 26, and on the bottom by buried layer 22. Floating body 24 may bethe portion of the original substrate 12 above buried layer 22 if buriedlayer 22 is implanted. Alternatively, floating body 24 may beepitaxially grown. Depending on how buried layer 22 and floating body 24are formed, floating body 24 may have the same doping as substrate 12 insome embodiments or a different doping, if desired in other embodiments.

Insulating layers 26 (for example, shallow trench isolation (STI)), maybe made of silicon oxide, for example, though other insulating materialsmay be used. Insulating layers 26 insulate cell 250 from neighboringcells 250 when multiple cells 250 are joined in an array 280 to make amemory device. The bottom of insulating layer 26 may reside inside theburied region 22 allowing buried region 22 to be continuous as shown inFIG. 23A. Alternatively, the bottom of insulating layer 26 may residebelow the buried region 22 as shown in the cross-sectional view ofanother embodiment of memory cell 250 in FIG. 23B. This requires ashallower insulating layer 28, which insulates the floating body region24, but allows the buried layer 22 to be continuous in the perpendiculardirection of the cross-sectional view shown in FIG. 23B. For simplicity,only memory cell 250 with continuous buried region 22 in all directionswill be shown from hereon.

A bit line region 16 having a second conductivity type, such as n-type,for example, is provided in floating body region 24 and is exposed atsurface 14. Bit line region 16 may be formed by an implantation processperformed on the material making up substrate 12, according to anyimplantation process known and typically used in the art. Alternatively,a solid state diffusion process could be used to form bit line region16.

A source line region 18 having a first conductivity type, such asp-type, for example, is also provided in floating body region 24 and isexposed at surface 14. Source line region 18 may be formed by animplantation process performed on the material making up substrate 12,according to any implantation process known and typically used in theart. Alternatively, a solid state diffusion process could be used toform source line region 18. The source line region 18 has the sameconductivity type as the floating body region 24, with the source lineregion 18 typically being more heavily doped than the floating bodyregion 24.

A gate 60 is positioned in between the bit line region 16 and sourceline region 18 and above the floating body region 24. The gate 60 isinsulated from floating body region 24 by an insulating layer 62.Insulating layer 62 may be made of silicon oxide and/or other dielectricmaterials, including high-K dielectric materials, such as, but notlimited to, tantalum peroxide, titanium oxide, zirconium oxide, hafniumoxide, and/or aluminum oxide. The gate 60 may be made of, for example,polysilicon material or metal gate electrode, such as tungsten,tantalum, titanium and/or one of their nitrides.

Memory cell 250 is asymmetric in that the conductivity type between thebit line region 16 and the source line region 18 is different. Theconductivity type of the source line region 18 is the same as that ofthe floating body 24, and as a result, the source line region 18 may beused to sense the potential of the floating body 24.

Cell 250 includes several terminals: word line (WL) terminal 70electrically connected to gate 60, bit line (BL) terminal 74electrically connected to bit line region 16, source line (SL) terminal72 electrically connected to source line region 18, buried well (BW)terminal 76 electrically connected to buried layer 22, and substrateterminal 78 is electrically connected to substrate 12. The SL terminal72 may not be shared across different cells 250 as it will electricallyshort floating body 24 region in multiple cells 250, hence precludingfloating body 24 to be used as charge storage region. As a result,arrays comprising memory cells 250 are typically limited to one or tworows only.

FIG. 23C illustrates an equivalent circuit representation of memory cell250, showing an intrinsic n-p-n bipolar device 230, formed by buriedwell region 22, floating body region 24, and bit line region 16, andgate 60 which is capacitively coupled to the floating body region 24.Source line region 18 is shown to be connected to the floating bodyregion 24.

FIG. 23D schematically illustrates a memory array 280 comprising 2 rowsof memory cells 250 according to an embodiment of the present invention.Present in FIG. 23D are word lines 70 a and 70 b, bit lines 74 a through74 p, buried well terminals 76 a and 76 b, substrate terminal 78, andsource line terminals 72 aa through 72 bp (the first and second indicesrefer to the row and column designation, respectively). Each of the wordlines 70 a and 70 b is associated with a single row of memory cells 250and is coupled to the gate 60 of each memory cell 250 in that row. Eachof the bit lines 74 a through 74 p is associated with a single column ofmemory cells 250 and is coupled to the bit line region 16 of each memorycell 250 in that column. The buried well terminals 76 a and 76 b may beassociated with a single row of memory cells 250 or may be commonthrough the memory array 280. Each of the source lines 72 aa through 72bp is associated with a single memory cell 250 in the memory array 280.

Several operations can be performed on memory cells 250 including:holding, read, write LOGIC-1 and write LOGIC-0 operations.

FIG. 24A schematically illustrates a cross-sectional view of memory cell250 and shows exemplary bias conditions applied thereto for performing aholding operation thereon. The holding operation follows the sameprinciple as that of memory cells 50 and 150 and may performed byapplying a positive back bias to the BW terminal 76, zero bias on the WLterminal 70, and BL terminal 74, while SL terminal 72 is left floating.The positive back bias applied to the buried layer region 22 connectedto the BW terminal 76 will maintain the state of the memory cell 250that it is connected to. The positive bias applied to the BW terminal 76needs to generate a sufficient electric field to trigger an impactionization mechanism as described with reference to the band diagramshown in FIGS. 4A and 4B. The impact ionization rate as a function ofthe electric field is for example described in Sze on pp. 37-41.

In one embodiment the bias conditions for the holding operation onmemory cell 250 are: 0.0 volts are applied to WL terminal 70, 0.0 voltsare applied to BL terminal 74, a positive voltage of about +1.2 volts isapplied to BW terminal 76, and 0.0 volts are applied to the substrateterminal 78, while the SL terminal 72 is left floating. In otherembodiments, different voltages may be applied to the various terminalsof memory cell 250.

In the holding operation described in FIG. 24, as well as in the arrayof FIG. 23D, there is no individually selected memory cell 250. Ratherthe holding operation is performed on all cells 250 connected to thesame buried well terminal 76.

FIG. 24B illustrates bias conditions for an alternative holdingoperation applied on a memory cell 250, as described in Widjaja-2. Theholding operation employs the principle of intrinsic SCR device formedby the substrate 12, the buried well region 22, the floating body region24, and the bit line region 16, as described in FIGS. 5D, 5E, and 17B.The holding operation may alternatively be performed by applying thefollowing bias conditions: zero voltage is applied to WL terminal 70 andBL terminal 74, a positive voltage is applied to the substrate terminal78, while the SL terminal 72 and BW terminal 76 are left floating. Inone particular non-limiting embodiment, a voltage of about 0.0 volts isapplied to BL terminal 74, a voltage of about 0.0 volts is applied to WLterminal 70, and about +1.2 volts is applied to terminal 78, while theSL terminal 72 and BW terminal 76 are left floating. However, thesevoltage levels may vary, while maintaining the relative relationshipstherebetween.

A read operation can be performed by directly sensing the potential ofthe floating body 24 through the SL terminal 72 connected to the sourceline region 18. If memory cell 250 is in LOGIC-1 state, a positivepotential, for example +0.6V-+0.8V, is stored in the floating body 24,while if memory cell 250 is in LOGIC-0 state, zero potential or lowpositive potential, for example 0-+0.2V, is stored in the floating body24. The maximum potential stored in the floating body 24 can bemodulated through the positive bias applied to the BW terminal 76. FIG.5C illustrates the charge stored in the floating body region 24 as afunction of the potential applied to the buried well region 22,connected to the BW terminal 76.

FIG. 25 is a schematic, cross-sectional illustration of a selectedmemory cell 250 a showing exemplary bias conditions that may be appliedto the selected memory cell 250 a to perform a write LOGIC-1 operationthereon. The write LOGIC-1 operation can be performed using aband-to-band tunneling mechanism, where the following bias conditionsare applied: a negative voltage is applied to the selected WL terminal70 a, a positive voltage is applied to the selected BL terminal 74 a,zero or positive voltage is applied to the selected BW terminal 76 a,and zero voltage is applied to the substrate terminal 78, while SLterminal 72 aa is left floating. The negative charge on the gate 60 andthe positive voltage on BL terminal 74 a create a strong electric field(for example, around 10⁶ V/cm in silicon, as described in Sze, p. 104)between the bit line region 16 and the floating body region 24 in theproximity of gate 60. This bends the energy band sharply upward near thegate 60 and bit line junction overlap region, causing electrons totunnel from the valence band to the conduction band, leaving holes inthe valence band. The electrons which tunnel across the energy bandbecome the drain leakage current, while the holes are injected intofloating body region 24 and become the hole charge that creates theLOGIC-1 state.

In one particular non-limiting embodiment, about −1.2 volts is appliedto the selected word line terminal 70 a, about +1.2 volts is applied tothe selected bit line terminal 74 a, about 0.0 volts or +1.2 volts isapplied to selected BW terminal 76 a, and about 0.0 volts is applied tosubstrate terminal 78, while SL terminal 72 aa is left floating. Thesevoltage levels are exemplary only may vary from embodiment toembodiment.

FIG. 26A is a schematic, cross-sectional illustration of a selectedmemory cell 250 a and exemplary bias conditions applied thereto toperform a write LOGIC-0 operation thereon. The write LOGIC-0 operationmay be performed by applying a negative voltage bias to the BL terminal74 a, a zero voltage bias to the WL terminal 70 a, zero or positivevoltage bias to the BW terminal 76 a, and zero voltage bias to thesubstrate terminal 78, while the SL terminal 72 aa is left floating.Under these conditions, the p-n junction between floating body 24 andbit line region 16 of the selected cell 250 is forward-biased,evacuating holes from the floating body 24. All memory cells 250 sharingthe same BL terminal 74 a will be written to simultaneously. To writearbitrary binary data to different memory cells 250, a write LOGIC-0operation is first performed on all the memory cells to be written,followed by one or more write LOGIC-1 operations on the memory cellsthat must be written to LOGIC-1.

In one particular non-limiting embodiment, about −1.2 volts is appliedto bit line terminal 74 a, about 0.0 volts is applied to WL terminal 70a, about 0.0 volts or +1.2 volts is applied to BW terminal 76 a, andabout 0.0 volts is applied to substrate terminal 78, while source lineterminal 72 aa is left floating. These voltage levels are exemplary onlyand may vary from embodiment to embodiment.

FIG. 26B is a schematic, cross-sectional illustration of a selectedmemory cell 250 a showing exemplary bias conditions that may be appliedthereto to perform a bit-selective write LOGIC-0 operation thereon. Thebit-selective write LOGIC-0 operation can be performed on memory cell250 a by applying a positive voltage to WL terminal 70 a, a negativevoltage to BL terminal 74 a, zero or positive voltage bias to the BWterminal 76 a, and zero voltage to the substrate terminal 78, while theSL terminal 72 aa is left floating. Under these conditions, the floatingbody 24 potential will increase through capacitive coupling from thepositive voltage applied to the WL terminal 70 a. As a result of thefloating body 24 potential increase combined with the negative voltageapplied to the BL terminal 74, the p-n junction between floating bodyregion 24 and bit line region 16 is forward-biased, evacuating holesfrom the floating body 24, thereby resulting in the LOGIC-0 state in thememory cell 250 a.

To reduce undesired write LOGIC-0 disturb to other memory cells 250 in amemory array, the applied potential can be optimized as follows: if thefloating body 24 potential of state LOGIC-1 is referred to as V_(FB1),then the voltage applied to the WL terminal 70 a is configured toincrease the floating body 24 potential by V_(FB1)/2 while −V_(FB1)/2 isapplied to BL terminal 74 a. Additionally, either ground or a slightlypositive voltage may also be applied to the BL terminals 74 ofunselected memory cells 250 that do not share the same BL terminal 74 aas the selected memory cell 250 a, while a negative voltage may also beapplied to the WL terminals 70 of unselected memory cells 250 that donot share the same WL terminal 70 a as the selected memory cell 250 a.

As illustrated in FIG. 26B, the following exemplary bias conditions maybe applied to the selected memory cell 250 a to perform a bit-selectivewrite LOGIC-0 operation: a potential of about −0.2 volts to BL terminal74 a, a potential of about +1.2 volts to the WL terminal 70 a, apotential of about +1.2 volts to the BW terminal 76 a, about 0.0 voltsto the substrate terminal 78, while the SL terminal 72 aa is leftfloating.

The transition between LOGIC-0 and LOGIC-1 states is defined by V_(TS)in FIGS. 5A and 5B. V_(TS) can be modulated by the potential differenceacross the emitter and collector terminals of the intrinsic n-p-nbipolar device 230 (see FIG. 23C); that is, between the BW terminal 76 aand the BL terminal 74 a. V_(TS) is inversely dependent on the potentialdifference between emitter and collector terminals (V_(CE)), as shown inFIG. 12C. The dependence of V_(TS) on V_(CE) can be utilized for thewrite LOGIC-0 operation. For example, the potential applied to the BWterminal 76 a can be reduced during the write LOGIC-0 operation, henceresulting in higher V_(TS), higher than the potential of the floatingbody region 24 V_(FB) of the selected memory cell 250 a during writeLOGIC-0 operation. Because the V_(FB) is now less than V_(TS), a netcurrent flowing out of the floating body region 24 will be observed.

Memory cell 250 may be used as a latch, where the SL terminal 72 can beconnected to the gate of another transistor, for example, to configureconnectivity of gates in a field programmable logic array (FPGA), asdescribed in FIG. 27. The SL terminal 72 of memory cell 250 is connectedto the gate of a switching transistor 82, which in turn connectsinterconnect lines 84 and 86. If the memory cell 250 is in LOGIC-1state, the floating body 24 will be positively charged, and the gate ofthe switching transistor 82 will be positively biased. If n-channelmetal-oxide-semiconductor (NMOS) transistor is used as the switchingtransistor 82 (as shown in FIG. 27), this will turn on the switchingtransistor 82 and connect the lines 84 and 86. If the memory cell 250 isin LOGIC-0 state, the floating body 24 will be neutrally charged, andthe switching transistor 82 will be turned off. As a result, noconnection between lines 84 and 86 is formed.

FIG. 28 illustrates an alternative arrangement of the use of memory cell250 as a configuration memory to configure connectivity in an FPGA,where an inverter 88 and a p-channel metal-oxide-semiconductor (PMOS)transistor 90 are used to restore the value of the signals passedbetween lines 84 and 86. This is because an NMOS switching transistor 82will only pass a maximum potential of about (V_(gs)−V_(th)), whereV_(gs) is the potential difference between the gate and the sourceterminals, and V_(th) is the threshold voltage of the NMOS transistor,respectively.

An electrical connection to the floating body region, such as thatbetween the SL terminal 72 to the floating body region 24 (through thesource line region 18) described in cell 250, can be used as a referencecell for reading a floating body memory cell, for example, as describedin Widjaja and Ranica, or the memory cells 50 and 150 according to thepresent invention.

FIG. 29A is a schematic, cross-sectional illustration of a memory cell250R1, which can be used as a reference cell in sensing the state of afloating body memory cell described in Widjaja and Ranica. Cell 250R1includes a substrate 12 of a first conductivity type such as p-type, forexample. Substrate 12 is typically made of silicon, but may also (oralternatively) comprise, for example, germanium, silicon germanium,gallium arsenide, carbon nanotubes, or other semiconductor materials. Insome embodiments of the invention, substrate 12 may be the bulk materialof the semiconductor wafer. In other embodiments, substrate 12 may be awell of the first conductivity type embedded in either a well of thesecond conductivity type or, alternatively, in the bulk of thesemiconductor wafer of the second conductivity type, such as n-type, forexample, (not shown in the figures). To simplify the description, thesubstrate 12 will usually be drawn as the semiconductor bulk material asit is in FIG. 29A.

A buried layer 22 of a second conductivity type such as n-type, forexample, is provided in the substrate 12. Buried layer 22 may be formedby an ion implantation process on the material of substrate 12.Alternatively, buried layer 22 may be grown epitaxially on top ofsubstrate 12.

A floating body region 24 of the first conductivity type, such asp-type, for example, is bounded on top by the surface 14, bit lineregion 16, source line region 18, sense line region 20, and insulatinglayer 62, on the sides by insulating layers 26, and on the bottom byburied layer 22. Floating body 24 may be the portion of the originalsubstrate 12 above buried layer 22 if buried layer 22 is implanted.Alternatively, floating body 24 may be epitaxially grown. Depending onhow buried layer 22 and floating body 24 are formed, floating body 24may have the same doping as substrate 12 in some embodiments or adifferent doping, if desired in other embodiments.

Insulating layers 26 (for example, shallow trench isolation (STI)), maybe made of silicon oxide, for example, though other insulating materialsmay be used. Insulating layers 26 insulate cell 250R1 from neighboringmemory cells, which include floating body memory cells described byWidjaja and Ranica or memory cells 50, 150 and 250 according to thepresent invention, or to neighboring reference cells 250R1 The bottom ofinsulating layer 26 may reside inside the buried region 22 allowingburied region 22 to be continuous as shown in FIG. 29A. Alternatively,the bottom of insulating layer 26 may reside below the buried region 22as shown in the cross-sectional view of another embodiment of memorycell 250R1 in FIG. 29B. This requires a shallower insulating layer 28(shown in dashed lines), which insulates the floating body region 24,but allows the buried layer 22 to be continuous in the perpendiculardirection of the cross-sectional view shown in FIG. 29B. For simplicity,only memory cell 250R1 with continuous buried region 22 in alldirections will be shown from hereon.

A bit line region 16 having a second conductivity type, such as n-type,for example, is provided in floating body region 24 and is exposed atsurface 14. Bit line region 16 may be formed by an implantation processperformed on the material making up substrate 12, according to anyimplantation process known and typically used in the art. Alternatively,a solid state diffusion process can be used to form bit line region 16.

A source line region 18 having a second conductivity type, such asn-type, for example, is provided in floating body region 24 and isexposed at surface 14. Source line region 18 may be formed by animplantation process performed on the material making up substrate 12,according to any implantation process known and typically used in theart. Alternatively, a solid state diffusion process can be used to formsource line region 18.

A sense line region 20 having a first conductivity type, such as p-type,for example, is also provided in floating body region 24 and is exposedat surface 14. Sense line region 20 may be formed by an implantationprocess performed on the material making up substrate 12, according toany implantation process known and typically used in the art.Alternatively, a solid state diffusion process can be used to form senseline region 20. The sense line region 20 has the some conductivity typeas the floating body region 24, with the sense line region 20 typicallybeing more heavily doped than the floating body region 24.

A gate 60 is positioned in between the bit line region 16 and sourceline region 18 and above the floating body region 24. The gate 60 isinsulated from floating body region 24 by an insulating layer 62.Insulating layer 62 may be made of silicon oxide and/or other dielectricmaterials, including high-K dielectric materials, such as, but notlimited to, tantalum peroxide, titanium oxide, zirconium oxide, hafniumoxide, and/or aluminum oxide. The gate 60 may be made of, for example,polysilicon material or metal gate electrode, such as tungsten,tantalum, titanium and/or their nitrides.

Memory cell 250R1 can be subdivided to include a region comprising afloating body memory cell 250S described in Widjaja and Ranica, wherethe floating body region 24 is used to store the states of the memorycell described by Widjaja and Ranica, or the memory cells 50, 150 and250 according to the present invention. This region 250S of thereference cell 250R1 is enclosed by dashed lines in FIGS. 29A and 29B.The sense line region 20 allows for an electrical connection to thefloating body region 24. The sense line region 20 is shown connected toa sense line terminal 73 in FIGS. 29A and 29B.

Floating body memory cells, including memory cells 50, 150 and 250according to the present invention, are typically read using a senseamplifier by comparing its property, for example, the current flowingfrom the BL terminal to the SL terminal of memory cells 50 or 150 tothat of a reference cell. Different reference cell schemes have beendisclosed, for example by averaging the cell currents of 128 LOGIC-1 and128 LOGIC-0 dummy cells as described in Ohsawa-2. Rather than averagingcell currents of multiple dummy cells with LOGIC-1 and logic-0, memorycell 250R1 can be used as a reference cell by applying an intermediatepotential (between the LOGIC-0 and LOGIC-1 states) to the floating bodyregion 24 through the sense region 20. For example, a positive voltagebias of +0.3V may be applied to the floating body region 24 (through thesense region 20). The resulting current flowing from the bit line region16 to the source line region 18 of the cell 250R1 will be in between thecell current of memory cell 50 or 150 in LOGIC-1 and LOGIC-0 states,similar to what is obtained by averaging LOGIC-1 and LOGIC-0 dummycells.

FIG. 30A illustrates a schematic, top view of a memory cell 250R2according to another embodiment of the present invention. Memory cell250R2 also provides an electrical connection to the floating body region24 through the sense region 20. The sense region 20 in this embodimentis located adjacent to the region similar to the floating body memorycells, for example, as described by Widjaja and Ranica, or memory cells50, 150 and 250 according to the present invention. FIGS. 30B and 30Care schematic, cross-section illustrations of cell 250R2 along the I-I′and II-II′ cut lines of FIG. 30A, respectively.

Cells 250R1 and 250R2 can also be used as reference cells during theholding operation of floating body memory cells, including memory cells50, 150 and 250 according to the present invention. Widjaja describes aholding or refresh method through the application of periodic pulses ofpositive voltage to the back bias terminal, for example, the BW terminal76.

FIG. 31 illustrates an algorithm 100 that may be employed to refresh thedata stored in floating body memory cells in parallel. At event 102, thestate of a reference cell, for example cell 250R1 or 250R2, is sensedand compared with a reference value. If the state of the reference cellis below the reference value, then a positive bias is applied to theback bias terminal to refresh the state of floating memory cells (forexample, memory cells 50, 150 or 250) at event 104. Since the refreshoperation is performed in parallel to all cells connected to the buriedwell terminal, the refresh operation can be performed in a fast manner.The refresh operation is self-select, with floating body memory cells50, 150 or 250 in LOGIC-0 state will remain in LOGIC-0 state, whilefloating body memory cells 50, 150 or 250 in LOGIC-1 state will remainin LOGIC-1 state. If the state of the reference cell is above thereference value, zero voltage is applied to the back bias terminal atevent 106. Following event 104 or 196, the state of the reference cellis compared again with the reference value, returning to event 102 andcontinuing to loop.

FIGS. 32A through 32E illustrate several implementations of thealgorithm 100. FIG. 32A shows a feedback loop based on a single-stageoperational amplifier (op-amp). The state of memory cell 250R1 is sensedthrough the sense region 20 connected to the sense terminal 73. If thepotential of floating body region 24 of memory cell 250R1 V_(FB) ishigher than the reference value V_(REF), then NMOS transistor 114 willconduct more current than NMOS transistor 112. Because only a fixedamount of current I_(TAIL) (determined by current source 120) isavailable, the current flowing through transistor 114, I₁₁₄, increases,and the current flowing through transistor 112, I₁₁₂, decreases. I₁₁₂ ismirrored by the current mirror constructed by PMOS transistors 116 and118, and acts to increase the back bias applied to the BW terminal 76,V_(DNWL), while I₁₁₄ acts to decrease V_(DNWL). Consequently, if V_(FB)is higher than V_(REF). this will decrease V_(DNWL), and conversely, ifV_(FB) is lower than V_(REF), V_(DNWL) will increase.

FIG. 32B shows another implementation of the algorithm 100 through aCMOS inverter 1130, comprising NMOS transistor 1132 and PMOS transistor1134. The input terminal 1130I of the inverter 1130 is connected to thesense region 20 of the cell 250R1, while the output voltage of theinverter is connected by output terminal 1130T to the BW terminal 76,V_(DNWL). FIG. 32C illustrates the input voltage-output voltagerelationship for the inverter 1130. If the floating body potentialV_(FB) is low, the output voltage V_(DNWL) will increase andsubsequently maintain the floating body potential V_(FB). If thefloating body potential V_(FB) is high, the output voltage V_(DNWL) willdecrease and subsequently reduce the floating body potential V_(FB).

FIG. 32D illustrates another implementation of the algorithm 100 with amixed-signal feedback loop. The potential of the floating body region 24V_(FB) (connected to the sense region 20) is digitized byanalog-to-digital converter (ADC) 1140 and sent into a digitalcontroller block 1142. The digital controller 1142 then compares thepotential of the floating body region 24 with a reference value V_(REF)and drives the buried well terminal 76 V_(DNWL) through adigital-to-analog converter (DAC) 1144.

FIG. 32E illustrates a schematic implementation of the mixed-signalfeedback loop shown in FIG. 32D. A 1-bit comparator block 1150 is usedto quantize the potential of the floating body region 24. The 1-bitcomparator block 1150 is typically referred to in the art as a StrongArmcomparator, for example described in “A 160 MHz, 32 b, 0.5 W CMOS RISCmicro-processor”, Montanaro et al., IEEE J. Solid-State Circuits, vol.31, no. 11, pp. 1703-1714, November 1996 (hereafter referred to as“Montanaro”). Both the digital controller block 1142 and the DAC block1144 may not be necessary provided the V_(dd) and the GND signals of theStrongArm comparator block 1150 is used as the input signal to theburied well terminal 76, V_(DNWL).

An example of the operation of the mixed-signal feedback loopillustrated in FIG. 32E is provided. When the clock signal CLK is low,the PMOS transistors 1152 and 1154 are switched on and pre-charge nodes1156 and 1158 to V_(dd). When the clock signal CLK switches to high,PMOS transistors 1152 and 1154 are now turned off, while NMOS transistor1160 is on and supplies current to two cross-coupled inverters (formedby NMOS transistor 1162 and PMOS transistor 1164, and NMOS transistor1166 and PMOS transistor 1168, respectively) through pseudo-differentialpair transistors 1170 and 1172.

If the potential of the floating body region 24 V_(FB) is higher thanthe reference value V_(REF), then more current will flow throughtransistor 1172 than through transistor 1170 and therefore the potentialof the node 1156 will decrease faster than the potential of the node1158. Since node 1156 is now at a lower potential than node 1158, NMOSdevice 1162 conducts less current than NMOS device 1166, and PMOS device1164 conducts more current than PMOS device 1168, reinforcing the growthof differential voltage between nodes 1156 and 1158. Eventually, node1158 reaches V_(dd) while node 1156 reaches ground GND.

If the potential of the floating body region 24 V_(FB) is lower than thereference value V_(REF), then more current flows through transistor 1170than through transistor 1172. Eventually, node 1156 will reach V_(dd)while node 1158 reaches ground GND.

Therefore, shortly after CLK signal transitions to high, the voltages atnodes 1156 and 1158 will result in a digital signal (V_(dd) or GND),indicating whether potential of the floating body region 24 V_(FB) isgreater than or less than the reference voltage V_(REF). When the CLKsignal transitions to low, both nodes 1156 and 1158 are pre-charged toV_(dd) again. To preserve the output state of the comparator during thistime, an SR-latch 1174, for example, as described in “Foundations ofDigital Logic Design”, Langholz, G., pp. 339-344, 1998 (which is herebyincorporated herein, in its entirety, by reference thereto, and isreferred to hereafter as “Langholz”) may be used.

Simplified waveforms associated with the circuit operation described inFIG. 32E are shown in FIG. 32F. As can be seen from the waveforms, evenwithout a digital controller block 1142 and DAC block 1144, the circuitmay operate as a simple bang-bang controller: if the compactor detectsthat the potential of the floating body region 24 V_(FB) is less thanthe reference potential V_(REF), then it drives V_(DNWL) to V_(dd),which subsequently increases the potential of the floating body region24 V_(FB). On the next CLK cycle, the comparison is performed again, andif, as shown in FIG. 32F, the V_(FB) has risen above V_(REF), V_(DNWL)remains at GND to reduce V_(FB).

Such method of holding the state of a memory cell may result in lowerpower consumption. This is, for example, compared to dynamic randomaccess memory (DRAM) refresh operation, which requires pre-charging thecorresponding bit lines, followed by essentially read-then-writeoperation of the refreshed DRAM memory cell.

Reference voltage V_(REF) may be generated in many different ways, forexample using a band gap reference, a resistor string, adigital-to-analog converter, etc. Similarly alternate voltage generatorsof types known in the art may be used.

At high temperature, the power consumed during the holding operationincreases due to the higher p-n junction leakage (e.g. formed betweenthe floating body region 24 and the bit line region 16) and also due tothe reduced impact ionization. The algorithm 100 shown in FIG. 31 may beemployed to reduce the holding operation power consumption at hightemperature. To maintain the states of the memory cells 50, 150, and250, the reference voltage V_(REF) needs to be higher than the V_(TS)(see FIGS. 5A and 5B). Since V_(TS) varies inversely with temperature,i.e. lower V_(TS) is observed at high temperature, the reference voltageV_(REF) may also be reduced. A band gap reference circuit may beconstructed to yield a reference voltage V_(REF) that varies inverselywith temperature.

FIG. 33 is a schematic, cross-sectional illustration of memory cell 350fabricated on a silicon-on-insulator (SOI) substrate. Memory cell 350includes a SOI substrate 212 of a first conductivity type such asp-type, for example. Substrate 212 is typically made of silicon, but mayadditionally or alternatively comprise, for example, germanium, silicongermanium, gallium arsenide, carbon nanotubes, or other semiconductormaterials. A buried insulator layer 222, such as buried oxide (BOX)layer, is provided above the substrate 212. The SOI substrate can beproduced either by an oxygen ion implantation process or through a waferbonding process. An overview of the SOI substrate fabrication isdescribed for example in “Frontiers of silicon-on-insulator”, Celler, G.K. and Cristoloveanu, S., J. App. Phys, vol. 93, no. 9, pp. 4955-4978,2003 (“Celler and Cristoloveanu”), which is hereby incorporated herein,in its entirety, by reference thereto.

A floating body region 224 of the first conductivity type, such asp-type, for example, is bounded on top by bit line region 216, sourceline region 218, and insulating layer 262; on the sides by insulatinglayers 226; and on the bottom by buried insulator layer 222. Floatingbody 224 may be the portion of the original substrate 212 above buriedinsulator layer 222. Floating body 224 may have the same doping assubstrate 212 in some embodiments or a different doping, if desired.

Insulating layers 226 (which may be, for example, shallow trenchisolation (STI)), may be made of silicon oxide, for example, thoughother insulating materials may be used. Insulating layers 226 insulatecell 350 from neighboring cells 350 when multiple cells 350 are joinedin an array to make a memory device.

A bit line region 216 having a second conductivity type, such as n-type,for example, is provided in floating body region 224 and is exposed atsurface 14. Bit line region 216 may be formed by an implantation processformed on the material making up substrate 212, according to anyimplantation process known and typically used in the art. Alternatively,a solid state diffusion process could be used to form bit line region216.

A source line region 218 having a first conductivity type, such asp-type, for example, is also provided in floating body region 224 and isexposed at surface 14. Source line region 218 may be formed by animplantation process formed on the material making up substrate 212,according to any implantation process known and typically used in theart. Alternatively, a solid state diffusion process could be used toform source line region 218. The source line region 218 has the sameconductivity type as the floating body region 224, with the source lineregion 218 typically being more heavily doped than the floating bodyregion 224.

A gate 260 is positioned in between the bit line region 216 and sourceline region 218 and above the floating body region 224. The gate 260 isinsulated from floating body region 224 by insulating layer 262.Insulating layer 262 may be made of silicon oxide and/or otherdielectric materials, including high-K dielectric materials, such as,but not limited to, tantalum peroxide, titanium oxide, zirconium oxide,hafnium oxide, and/or aluminum oxide. The gate 260 may be made of, forexample, polysilicon material or metal gate electrode, such as tungsten,tantalum, titanium and/or their nitrides.

Memory cell 350 is asymmetric in that the conductivity type between thebit line region 216 and the source line region 218 is different. Theconductivity type of the source line region 218 is the same as that ofthe floating body 224, and as a result, the source line region 218 maybe used to sense the potential of the floating body 224.

Cell 350 includes several terminals: word line (WL) terminal 270electrically connected to gate 260, bit line (BL) terminal 274electrically connected to bit line region 216, source line (SL) terminal272 electrically connected to source line region 218, and substrateterminal 278 electrically connected to substrate 212. The SL terminal272 may not be shared across different cells 350 as it will electricallyshort floating body 224 region in multiple cells 350, hence precludingfloating body 224 to be used as charge storage region. As a result,arrays comprising memory cells 350 are typically limited to one or tworows only.

The operation of memory cell 350 is similar to that of memory cell 250,except that a holding operation employing a back bias terminal does notapply due to the absence of back bias terminal on memory cell 350. As aresult, periodic refresh operations may need to be performed on memorycell 350 to maintain the state of the floating body 224. The refreshoperation on memory cell 350 may be performed by first reading the stateof the floating body 224 by directly sensing the floating body 224potential through the source line region 218. If the memory cell 350 isin LOGIC-1 state, then a write LOGIC-1 operation is performed on thecorresponding cell 350. If the memory cell 350 is in LOGIC-0 state, thena write LOGIC-0 operation can be performed on the corresponding cell350. Alternatively, if the memory cell 350 is in LOGIC-0 state, nofurther write operation is needed on memory cell 350.

A reference cell 350R, which for example, can be used during a readoperation, may also be constructed on an SOI substrate. FIG. 34Aillustrates a schematic, top view of memory cell 350R, which alsoprovides an electrical connection to the floating body region 224through the sense region 220. The sense region 220 is now locatedadjacent to the region similar to a floating body memory cellconstructed in an SOI substrate described for example by Okhonin.However, the sense region 220 is now electrically connected to thefloating body region 224. FIGS. 34B and 34C show the cross-sectionalviews of memory cell 350R along the I-I′ and II-II′ cut lines,respectively.

FIGS. 35A through 35C show alternative embodiments of memory cell 250,comprising a three-dimensional memory structure. In this embodiment,memory cell 250 has a fin structure 52 (see FIGS. 35B-35C) extendingsubstantially perpendicular to, and above the top surface of thesubstrate 12. Fin structure 52 is conductive and may be built on buriedwell layer 22 or buried insulator 22. If fin structure 52 is built onburied well layer 22, it may be formed by an ion implantation process onthe material of substrate 12 or grown epitaxially. Buried well layer orburied insulator layer 22 insulates the floating substrate region 24,which has a first conductivity type (such as p-type conductivity type),from the bulk substrate 12 having a first conductivity type (such asp-type conductivity type).

Fin structure 52 includes bit line region 16 of a second conductivitytype (such as n-type conductivity type) and source line region 18 of afirst conductivity type (such as p-type conductivity type). Bit lineregion 16 may be formed by an implantation process formed on thematerial making up substrate 12, according to any implantation processknown and typically used in the art. Alternatively, a solid statediffusion process could be used to form bit line region 16.

A source line region 18 having a first conductivity type, such asp-type, for example, is also provided in floating body region 24. Sourceline region 18 may be formed by an implantation process formed on thematerial making up substrate 12, according to any implantation processknown and typically used in the art. Alternatively, a solid statediffusion process could be used to form source line region 18. Thesource line region 18 has the same conductivity type as the floatingbody region 24, with the source line region 18 typically being moreheavily doped than the floating body region 24.

Cell 250 further includes gates 60 on two opposite sides of the floatingsubstrate region 24 as shown in FIG. 35B. Alternatively, gate 60 canenclose three sides of the floating substrate region 24 as shown in FIG.35C. Gates 60 are insulated from floating body 24 by insulating layers62. Gates 60 are positioned between the first and second regions 16, 18,adjacent to the floating body 24.

Memory cell 250 is asymmetric in that the conductivity type between thebit line region 16 and the source line region 18 is different. Theconductivity type of the source line region 18 is the same as that ofthe floating body 24, and as a result, the source line region 18 may beused to sense the potential of the floating body 24.

Cell 250 includes several terminals: word line (WL) terminal 70electrically connected to gate 60, bit line (BL) terminal 74electrically connected to bit line region 16, source line (SL) terminal72 electrically connected to source line region 18, buried well (BW)terminal 76 electrically connected to buried well layer 22, andsubstrate terminal 78 electrically connected to substrate 12. The SLterminal 72 may not be shared across different cells 250 as it willelectrically short floating body 24 region in multiple cells 250, henceprecluding floating body 24 to be used as charge storage region. As aresult, arrays comprising of memory cell 250 are typically limited toone or two rows only.

Similarly, three-dimensional embodiments of memory cells 50 and 150 andreference cells 250R1 and 250R2 may be constructed in a similar mannerFIGS. 36A through 36C illustrate cell 250R2 having a fin structure 52(see FIGS. 36B-36C) extending substantially perpendicular to, and abovethe top surface of the substrate 12. Fin structure 52 is conductive andmay be built on buried well layer 22 or buried insulator 22. If finstructure 52 is built on buried well layer 22, it may be formed by anion implantation process on the material of substrate 12 or grownepitaxially. Buried well layer or buried insulator layer 22 insulatesthe floating substrate region 24, which has a first conductivity type(such as p-type conductivity type), from the bulk substrate 12 having afirst conductivity type (such as p-type conductivity type).

Fin structure 52 includes bit line region 16 of a second conductivitytype (such as n-type conductivity type) and source line region 18 of asecond conductivity type (such as n-type conductivity type). Bit lineregion 16 and source line region 18 may be formed by an implantationprocess formed on the material making up substrate 12, according to anyimplantation process known and typically used in the art. Alternatively,a solid state diffusion process could be used to form bit line region 16and source line region 18.

Fin structure 52 also includes sense region 20 of a first conductivitytype (such as p-type conductivity type). Sense region 20 may be formedby an implantation process formed on the material making up substrate12, according to any implantation process known and typically used inthe art. Alternatively, a solid state diffusion process could be used toform sense region 20. The sense region 20 is now electrically connectedto the floating body region.

Another embodiment of a method to increase the read signal of floatingbody memory cells, such as memory cells 50, 150, and 250 according tothe present invention, is to increase the amount of charge stored in thefloating body region 24. To maintain or increase the amount of chargestored in the floating body 24, it may be necessary to increase thedepth of the floating body 24. This may be accomplished by a deeperburied well region 22 as well as deeper insulator region 26. The depthof the insulator region 26 may be constrained by the etch process neededto form the trench, which later on forms the insulator region 26. Amethod of processing floating body memory cells (for example memorycells 50, 150, and 250 along with floating body memory cells describedby Widjaja, Ranica, and Okhonin) with increased floating body 24 depthsis described with reference to FIGS. 37A through 37G using memory cell250 as an example. FIGS. 37A through 37G show schematic, cross-sectionalviews of memory cells 250 at various stages in the manufacturingprocess.

FIG. 37A illustrates the early steps of the process. An ion implantationis performed to form the buried well region 22 in the substrate 12 ofthe memory cell.

Referring to FIG. 37B, in an exemplary 130 nanometer (nm) process, athin silicon oxide layer 302 with a thickness of about 100 A may begrown on the surface of substrate 12. This may be followed by adeposition of about 200 A of polysilicon layer 304. This in turn may befollowed by deposition of about 1200 A silicon nitride layer 306. Otherprocess geometries like, for example, 250 nm, 180 nm, 90 nm, 65 nm,etc., may be used. Similarly, other numbers of, thicknesses of, andcombinations of protective layers 302, 304 and 306 may be used inrespect to the change in process geometries and/or other factors.

FIG. 37C illustrates a pattern opening the areas to become trench 308 isformed using a lithography process. Then the silicon oxide 302,polysilicon 304, silicon nitride 306 layers may be subsequentlypatterned using the lithography process and then may be etched, followedby a silicon etch process, creating trench 308. In an exemplary 130 nmprocess, the trench 208 depth may be about 1000 A. Other processgeometries including, but not limited to 250 nm, 180 nm, 90 nm, 65 nm,etc., may be used. Similarly, other trench depths may be used.

As shown in FIG. 37D, subsequent to the formation of trench 308, an ionimplantation is performed to form a region 23 of a second conductivitytype (e.g. n-type conductivity). Multiple ion implantations withdifferent energies may be performed to extend the depth of the region23.

As shown in FIG. 37E, this may be followed by a silicon oxidation orsilicon oxide deposition step, which will grow or deposit silicon oxidefilms in trench 308 which will become insulating layer 26. In anexemplary 130 nm process, about 2000 A silicon oxide may be grown ordeposited. The silicon nitride layer 306 and the polysilicon layer 304may then be removed which may then be followed by a wet etch process toremove silicon oxide layer 302 (and a portion of the silicon oxide filmsof the insulator layer 26). Other process geometries including, but notlimited to: 250 nm, 180 nm, 90 nm, 65 nm, etc., may be used. Similarly,other insulating layer materials, heights, and thicknesses as well asalternate sequences of processing steps may be used.

FIG. 37F shows results of a subsequent oxidation step to form the gateinsulator 62 followed by polysilicon deposition to form the gate 60. Thepolysilicon gate 60 is subsequently patterned and etched.

As shown in FIG. 37G, another ion implantation step may then beperformed to form the bit line region 16 of a second conductivity type(e.g. n-type conductivity) and source line region 18, which is of afirst conductivity type (e.g. p-type conductivity) in memory cell 250.This may then be followed by backend processing to form contact andmetal layers (not shown in FIGS. 37A through 37G). In this and manysubsequent figures, gate layer 60 and gate insulating layer 62 are shownflush with the edge of insulating layer 26. In some embodiments, gatelayer 60 and gate insulating layer 62 may overlap insulating layer 26 toprevent any of the implant dopant for bit line region 16 and source lineregion 18 from inadvertently implanting between gate layer 60 and gateinsulating layer 62 and the adjacent insulating layer 26.

FIG. 38A is a schematic, cross-sectional illustration of memory cell 450fabricated on a silicon-on-insulator (SOI) substrate. Memory cell 450includes a SOI substrate 212 of a first conductivity type such asp-type, for example. Substrate 212 is typically made of silicon, but mayadditionally or alternatively comprise, for example, germanium, silicongermanium, gallium arsenide, carbon nanotubes, or other semiconductormaterials. A buried insulator layer 222, such as buried oxide (BOX)layer, is provided above the substrate 212. The SOI substrate can beproduced either by oxygen ion implantation process or through waferbonding process. An overview of the SOI substrate fabrication isdescribed for example in Celler and Cristoloveanu, which is herebyincorporated herein, in its entirety, by reference thereto. A floatingbody region 224 of the first conductivity type, such as p-type, forexample, is bounded on top by the surface 214, source line region 218,and insulating layer 262; on the sides by insulating layers 226 andsource line region 218; and on the bottom by buried insulator layer 222.Floating body 224 may be the portion of the original substrate 212 aboveburied insulator layer 222. Floating body 224 may have the same dopingas substrate 212 in some embodiments or a different doping, if desired.

Insulating layers 226 (which may be, for example, shallow trenchisolation (STI)), may be made of silicon oxide, for example, thoughother insulating materials may be used. Insulating layers 226 insulatecell 450 from neighboring cells 450 when multiple cells 450 are joinedin an array to make a memory device.

A region 218 having a second conductivity type, such as n-type, forexample, is provided in floating body region 224 and is exposed atsurface 214. Region 218 may be formed by an implantation process formedon the material making up substrate 212, according to any implantationprocess known and typically used in the art. Alternatively, a solidstate diffusion process could be used to form region 218.

A source line region 220 having a first conductivity type, such asp-type, for example, is also provided in floating body region 224 and isexposed at surface 214. Source line region 220 may be formed by animplantation process formed on the material making up substrate 212,according to any implantation process known and typically used in theart. Alternatively, a solid state diffusion process could be used toform source line region 220. The source line region 220 has the sameconductivity type as the floating body region 224, with the source lineregion 218 typically being more heavily doped than the floating bodyregion 224.

The source line region 220 is electrically connected to source line (SL)terminal 272 through a conductive material 271. The conductive material271 may be made of, for example, polysilicon material, or metalelectrode, such as tungsten, aluminum, and/or copper. The conductivematerial 271 forms an ohmic contact 213 with the source line region 220.

The conductive material 273 forms a contact with the floating bodyregion 224. The conductive material 73 may be made of, for example,metal electrode, such as tungsten or aluminum, or metal silicides, suchas nickel silicide or platinum silicide. In contrast to the ohmiccontact between conductive material 271 and source line region 220,conductive material 273 forms a Schottky contact 215 with the floatingbody region 224.

A gate 260 is positioned in between the conductive material 273 (and theSchottky contact 215) and region 218 and above the floating body region224. The gate 260 is insulated from floating body region 224 byinsulating layer 262. Insulating layer 262 may be made of silicon oxideand/or other dielectric materials, including high-K dielectricmaterials, such as, but not limited to, tantalum peroxide, titaniumoxide, zirconium oxide, hafnium oxide, and/or aluminum oxide. The gate260 may be made of, for example, polysilicon material or metal gateelectrode, such as tungsten, tantalum, titanium and/or their nitrides.

Cell 450 includes several terminals: word line (WL) terminal 270electrically connected to gate 260, bit line (BL) terminal 274electrically connected to conductive material 273, source line (SL)terminal 272 electrically connected to source line region 220 (throughthe conductive material 271), and substrate terminal 278 electricallyconnected to substrate 212.

FIGS. 38B and 38C show two equivalent schematic representation of memorycell 450. As shown in FIG. 38B, inherent in memory cell 450 areback-to-back p-n diode 230 a, formed by source line region 220 andregion 218, and Schottky diode 230 b, formed by floating body 224 andconductive material 273. Another equivalent circuit representation isshown in FIG. 38C, showing the interconnected p-n-p bipolar device 230 c(formed by source line region 220, region 218, and floating body 224)and n-p-m bipolar device 230 d (formed by region 218, floating body 224,and the conductive material 273. The state of the memory cell 450 isstored in the floating region 224.

FIG. 38D schematically illustrates a memory array 480 comprising ofmemory cells 450 according to an embodiment of the present invention.Present in FIG. 38D are word lines 270 a, 270 b, through 270 n, bitlines 274 a, 274 b, through 274 p, source line terminals 272 a, 272 b,through 272 n, while a common substrate terminal 278 is not shown inFIG. 38D. Each of the word lines 270 a, 270 b, through 270 n isassociated with a single row of memory cells 450 and is coupled to thegate 260 of each memory cell 450 in that row. Each of the bit lines 274a through 274 p is associated with a single column of memory cells 450and is coupled to the conductive region 273 of each memory cell 450 inthat column Each of the source lines 272 a through 272 n is associatedwith a single row of memory cells 450 and is coupled to the source lineregion 220 of each memory cell 450 in that row.

Several operations can be performed on memory cells 250 including: read,write LOGIC-1 and write LOGIC-0 operations. Examples of memoryoperations employing interconnected p-n-p and n-p-n bipolar devices,often referred to as silicon controlled rectifier (SCR), are given forexample in “A novel capacitor-less DRAM cell using ThinCapacitively-Coupled Thyristor (TCCT)”, Cho H.-J., et al., pp. 311-314,Tech Digest, 2005 International Electron Devices Meeting, December, 2005(“Cho”); in U.S. Pat. No. 6,229,161 “Semiconductor Capacitively-CoupledNDR Device and Its Applications in High-Density High-Speed Memories andin Power Switches”, Nemati F. and Plummer J. D. (“Nemati-1”); in U.S.Pat. No. 6,653,175 “Stability in Thyristor-Based Memory Device”, NematiF. et al. (“Nemati-2”), which are incorporated herein, in theirentireties, by reference thereto.

FIG. 39A schematically illustrates bias conditions applied to the memoryarray 480 to perform a read operation, according to an embodiment of thepresent invention, while FIG. 39B schematically illustrates biasconditions applied on an exemplary selected memory cell 450 a from array480 in FIG. 39A. Any sensing scheme known in the art can be used withmemory cell 450, including, for example, the sensing schemes disclosedby Ohsawa-1 and Ohsawa-2, which are each incorporated by referenceherein in their entireties.

The read operation can be performed by applying the following biasconditions: a negative voltage is applied to the selected WL terminal270 a, zero voltage is applied to the selected BL terminal 274 a, apositive voltage is applied to the selected SL terminal 272 a, and zerovoltage is applied to the substrate terminal 278. If the cell 450 is ina LOGIC-1 state having holes in the floating body region 224, theintrinsic p-n-p-m SCR device will be turned on and a higher cell currentis observed compared to when cell 450 is in a LOGIC-0 state having noholes in the floating body region 224.

In one particular non-limiting embodiment, about −1.2 volts is appliedto the selected word line terminal 270 a, about 0.0 volts is applied tothe selected bit line terminal 274 a, about +1.2 volts is applied toselected SL terminal 272 a, and about 0.0 volts is applied to substrateterminal 278. These voltage levels are exemplary only may vary fromembodiment to embodiment. The voltage bias applied to the SL terminal272 a needs to be greater than the voltage bias applied to the BLterminal 274 a, while the voltage applied to the WL terminal 270 a needsto be kept low to avoid writing the selected memory cell 450 a.

FIG. 40A schematically illustrates bias conditions applied to the memoryarray 480 to perform a write LOGIC-1 operation according to anembodiment of the present invention, while FIG. 40B schematicallyillustrates bias conditions applied on an exemplary selected memory cell450 a from array 480 in FIG. 40A.

The write LOGIC-1 operation can be performed by applying the followingbias conditions: a positive voltage is applied to the selected WLterminal 270 a, zero voltage is applied to the selected BL terminal 274a, a positive voltage is applied to the selected SL terminal 272 a,while zero voltage is applied to the substrate terminal 278. Thepositive voltage applied to the WL terminal 270 a will increase thepotential of the floating body 224 through capacitive coupling andcreate a feedback process that turns the SCR device on. Once the SCRdevice of cell 450 is in conducting mode (i.e., has been “turned on”)the SCR becomes “latched on” and the voltage applied to WL terminal 270can be removed without affecting the “on” state of the SCR device.

In one particular non-limiting embodiment, a voltage of about 0.0 voltsis applied to terminal 274 a, a voltage of about +1.2 volts is appliedto terminal 270 a, about +1.2 volts is applied to terminal 272 a, andabout 0.0 volts is applied to substrate terminal 278. However, thesevoltage levels may vary, while maintaining the relative relationshipsbetween the voltages applied, as described above, e.g., the voltageapplied to terminal 272 remains greater than the voltage applied toterminal 274 and a positive voltage applied to the WL terminal 270 toincrease the potential of the floating body 224 through capacitivecoupling.

FIG. 41A schematically illustrates bias conditions applied to the memoryarray 480 to perform a write LOGIC-0 operation according to anembodiment of the present invention, while FIG. 41B schematicallyillustrates bias conditions applied on an exemplary selected memory cell450 a from array 480 in FIG. 41A.

The write LOGIC-0 operation can be performed by applying the followingbias conditions: a positive voltage is applied to the selected WLterminal 270 a, zero voltage is applied to the selected BL terminal 274a, zero voltage is applied to the selected SL terminal 272 a, while zerovoltage is applied to the substrate terminal 278. Under these conditionsthe voltage difference between anode and cathode, defined by thevoltages at SL terminal 272 and BL terminal 274, will become too smallto maintain the SCR device in conducting mode. As a result, the SCRdevice of cell 450 will be turned off.

In one particular non-limiting embodiment, a voltage of about 0.0 voltsis applied to terminal 274 a, a voltage of about +1.2 volts is appliedto terminal 270 a, and about 0.0 volts is applied to terminal 272 a,while about 0.0 volts is applied to substrate terminal 278. However,these voltage levels may vary, while maintaining the relativerelationships between the voltages applied, as described above, e.g.,that the voltage difference between the SL terminal 272 and BL terminal274 is low enough to maintain the SCR device in conducting mode.

FIG. 42A is a schematic, cross-sectional illustration another embodimentof memory cell 550 incorporating intrinsic p-n-p-m device fabricated ona bulk semiconductor substrate. Memory cell 550 includes a substrate 12of a first conductivity type such as p-type, for example. Substrate 12is typically made of silicon, but may additionally or alternativelycomprise, for example, germanium, silicon germanium, gallium arsenide,carbon nanotubes, or other semiconductor materials.

A floating body region 24 of the first conductivity type, such asp-type, for example, is bounded on top by the surface 14, region 18, andinsulating layer 62, on the sides by insulating layers 26, and on thebottom by buried layer 22. Floating body 24 may be the portion of theoriginal substrate 12 above buried layer 22 if buried layer 22 isimplanted. Alternatively, floating body 24 may be epitaxially grown.Depending on how buried layer 22 and floating body 24 are formed,floating body 24 may have the same doping as substrate 12 in someembodiments or a different doping, if desired in other embodiments.

Insulating layers 26 (like, for example, shallow trench isolation(STI)), may be made of silicon oxide, for example, though otherinsulating materials may be used. Insulating layers 26 insulate cell 50from neighboring cells 50 when multiple cells 50 are joined in an arrayto make a memory device. The bottom of insulating layer 26 may resideinside the buried region 22 allowing buried region 22 to be continuousas shown in FIG. 42A. Alternatively, the bottom of insulating layer 26may reside below the buried region 22 as shown in the cross-sectionalview of another embodiment of memory cell 550 in FIG. 42B. This requiresa shallower insulating layer 28 (shown in dashed lines), which insulatesthe floating body region 24, but allows the buried layer 22 to becontinuous in the perpendicular direction of the cross-sectional viewshown in FIG. 42B. For simplicity, only memory cell 550 with continuousburied region 22 in all directions will be shown from hereon.

A region 18 having a second conductivity type, such as n-type, forexample, is provided in floating body region 24, so as to bound aportion of the top of the floating body region in a manner discussedabove, and is exposed at surface 14. Region 18 may be formed by animplantation process on the material making up substrate 12, accordingto any implantation process known and typically used in the art.Alternatively, a solid state diffusion process could be used to formregion 18.

A source line region 20 having a first conductivity type, such asp-type, for example, is also provided above the surface 14. Source lineregion 20 may be formed by an epitaxial growth process, according to anyepitaxial growth process known and typically used in the art, forexample as described in “Low voltage/Sub-ns Operation BulkThyristor-SRAM (BT-RAM) Cell with Double Selective Epitaxy Emitters(DEE)”, Sugizaki, T. et al., IEEE Symposium on VLSI Technology 2007, pp.170-171, June 2007 (“Sugizaki”), which is incorporated herein, in itsentirety, by reference thereto.

The source line region 20 is electrically connected to source line (SL)terminal 72 through a conductive material 71. The conductive material 71may be made of, for example, polysilicon material, or metal electrode,such as tungsten, aluminum, and/or copper. The conductive material 71forms an ohmic contact 13 with the source line region 18.

The conductive material 73 forms a contact with the floating body region24. The conductive material 73 may be made of, for example, metalelectrode, such as tungsten or aluminum, or metal silicides, such asnickel silicide or platinum silicide. In contrast to the ohmic contactbetween conductive material 71 and source line region 20, conductivematerial 73 forms a Schottky contact 15 with the floating body region24.

A gate 60 is positioned in between the bit line region 16 and sourceline region 18 and above the floating body region 24. The gate 60 isinsulated from floating body region 24 by an insulating layer 62.Insulating layer 62 may be made of silicon oxide and/or other dielectricmaterials, including high-K dielectric materials, such as, but notlimited to, tantalum peroxide, titanium oxide, zirconium oxide, hafniumoxide, and/or aluminum oxide. The gate 60 may be made of, for example,polysilicon material or metal gate electrode, such as tungsten,tantalum, titanium and their nitrides.

Cell 50 includes several terminals: word line (WL) terminal 70electrically connected to gate 60, bit line (BL) terminal 74electrically connected to bit line region 16, source line (SL) terminal72 electrically connected to source line region 20, buried well (BW)terminal 76 electrically connected to buried layer 22, and substrateterminal 78 electrically connected to the substrate 12.

The read and write operations of the memory cell 550 is similar to thoseof memory cell 450. A holding operation may also be performed on memorycell 550 through the application of a positive voltage on the BWterminal 72, similar to the holding operation performed on memory cell50 described in FIGS. 3A, 3B, 4A, and 4B; holding operation performed onmemory cell 150 described in FIG. 17A; and holding operation performedon memory cell 250 described in FIG. 24A.

Alternatively, holding operation employing the intrinsic SCR device,formed by the substrate 12, buried well 22, floating body region 24, andconductive material 73 forming Schottky contact 15 with the floatingbody region 24, as described in FIGS. 5D and 5E, 17B, and 24B may alsobe performed on memory cell 550.

While the present invention has been described with reference to thespecific embodiments thereof, it should be understood by those skilledin the art that various changes may be made and equivalents may besubstituted without departing from the true spirit and scope of theinvention. In addition, many modifications may be made to adapt aparticular situation, material, composition of matter, process, processstep or steps, to the objective, spirit and scope of the presentinvention. All such modifications are intended to be within the scope ofthe claims appended hereto.

For example, the first and second conductivity types may be reversed andthe applied voltage polarities inverted while staying within the scopeof the present invention.

While many different exemplary voltage levels were given for variousoperations and embodiments, these may vary from embodiment to embodimentwhile staying within the scope of the present invention.

The invention may be manufactured using any process technology at anyprocess geometry or technology node and be within the scope of theinvention. Further, it should be understood that the drawing figures arenot drawn to scale for ease of understanding and clarity ofpresentation, and any combination of layer composition, thickness,doping level, materials, etc. may be used within the scope of theinvention.

While exemplary embodiments typically showed a single memory array forthe purpose of simplicity in explaining the operation of the variousmemory cells presented herein, a memory device employing the memorycells of the presentation may vary in many particulars in terms ofarchitecture and organization while staying within the scope of theinvention. Such embodiments may, without limitation, include featuressuch as multiple memory arrays, segmentation of the various controllines with or without multiple levels of decoding, simultaneouslyperforming multiple operations in multiple memory arrays or in the samearrays, employing many different voltage or current sensing circuits toperform read operations, using a variety of decoding schemes, using morethan one type of memory cell, employing any sort of interface tocommunicate with other circuitry, and/or employing many differentcircuits known in the art to generate voltage or currents for use inperforming the various operations on the memory array or arrays. Suchcircuits may without limitation include, for example, digital-to-analogconverters (DACs), analog-to-digital converters (ADCs), operationalamplifiers (Op Amps), comparators, voltage reference circuits, currentmirrors, analog buffers, etc.

That which is claimed is:
 1. An integrated circuit comprising: asemiconductor memory array comprising a plurality of asymmetricbi-stable semiconductor memory cells arranged in a matrix of rows andcolumns, wherein each said asymmetric bi-stable semiconductor memorycell includes: a first bipolar device having a first floating baseregion, a first emitter, and a first collector; and a second bipolardevice having a second floating base region, a second emitter, and asecond collector, wherein said first floating base region and saidsecond floating base region comprise a common floating base region andwherein said common floating base region comprises means for storing acharge or lack of charge as a volatile memory indicative of a state ofthe asymmetric semiconductor memory cell; wherein said first collectoris common to said second collector; wherein at least one of said firstbipolar device or second bipolar device maintains a state of said memorycell, and wherein performance characteristics of said first bipolardevice are different from performance characteristics of said secondbipolar device; wherein one of said first emitter or second emittercomprises an electrode electrically connected to said first floatingbase region and second floating base region, wherein said electrodeforms a Schottky contact with said first floating base region and secondfloating base region; and a control circuit configured to perform aholding operation on said array.
 2. The integrated circuit of claim 1,wherein said asymmetric bi-stable semiconductor memory cell furthercomprises a gate positioned above said common floating base region. 3.The integrated circuit of claim 2, wherein said asymmetric bi-stablesemiconductor memory cell further comprises: a substrate; and a buriedlayer; wherein said buried layer comprises said first and secondcollectors; and wherein said substrate is separated from said commonfloating base region by said buried layer.
 4. The integrated circuit ofclaim 2, wherein said asymmetric bi-stable semiconductor memory cellfurther comprises a gap region on a surface of said common floating baseregion, said gap region located between one of said first bipolar deviceor second bipolar device and said gate.
 5. The integrated circuit ofclaim 3, further comprising: a word line terminal electrically connectedto said gate; a bit line terminal electrically connected to said firstemitter; a source line terminal electrically connected to said secondemitter; a buried well terminal electrically connected to said buriedlayer; and a substrate terminal electrically connected to saidsubstrate.
 6. The integrated circuit of claim 2, wherein said asymmetricbi-stable semiconductor memory cell further comprises an insulatinglayer insulating said gate from said common floating base region.
 7. Theintegrated circuit of claim 1, wherein: said common floating base regionhas a first conductivity type selected from a p-type conductivity typeand an n-type conductivity type; said first collector and said secondcollector have a second conductivity type selected from said p-typeconductivity type and said n-type conductivity type, said secondconductivity type being different from said first conductivity type; anda second region having said second conductivity type, said second regionbeing separated from at least one of said first and second collectors bysaid common floating base region.
 8. The integrated circuit of claim 1,wherein said asymmetric bi-stable semiconductor memory cell has a finstructure.
 9. The integrated circuit of claim 1, wherein said controlcircuit provides electrical signals to at least two of said memory cellsto maintain the states of said at least two memory cells.
 10. Theintegrated circuit of claim 9, wherein said electrical signals areconstant voltage biases.
 11. The integrated circuit of claim 9, whereinsaid electrical signals are periodic pulses of voltage.
 12. Theintegrated circuit of claim 9, wherein said electrical signals result inat least two stable states.
 13. The integrated circuit of claim 1,wherein said control circuit comprises a voltage generator circuit. 14.The integrated circuit of claim 13, further comprising a multiplexerelectrically connected between said voltage generator circuit and saidfirst and second collectors.
 15. The integrated circuit of claim 1,further comprising a semiconductor memory cell connected to at least oneof said asymmetric bi-stable semiconductor memory cells, saidsemiconductor memory cell comprising: a floating body region having afirst conductivity type selected from n-type conductivity type andp-type conductivity type; said floating body region comprising means forstoring a charge or lack of charge indicative of a state of thesemiconductor memory cell; a first region having said first conductivitytype and being in direct contact with said floating body region; asecond region in electrical contact with said floating body region andspaced apart from said first region, said second region having a secondconductivity type selected from said n-type and said p-type conductivitytype, said second conductivity type being different from said firstconductivity type.
 16. The integrated circuit of claim 15, wherein saidsemiconductor memory cell is useable as a reference cell by applying anintermediate potential between a first potential indicative of a logic-0state and second potential indicative of a logic-1 state to saidfloating body region through said first region.
 17. The integratedcircuit of claim 15, wherein said semiconductor memory cell isconfigured for use as a reference cell, further comprising a thirdregion in electrical contact with said floating body region, said thirdregion having said second conductivity type.